I have been called for, it appears.
Canonically, of course, every origin or background of Asmodeus is a lie of some kind. While it's definitely accurate that he fell into Hell from elsewhere, and reasonably likely that his most well-known appearance is not the true nature of him, no one account best encapsulates the reality of what the Lord Below is.
That said, if in your campaign you want both of those stories to be true, here's what I would recommend:
The forces that Asmodeus commanded as a divine champion against chaos were not the forces of good; the Fiendish Codex makes abundantly clear that he was a champion of Law. If weakened Jazirian retreated and became a "god," it follows that weakened Ahriman might have hidden as a servant of these "gods," to discover their natures and find a means to undo them. The true nature of him would have been irrelevant, for Ahriman was indeed a great entity of Law and in his heavenly aspect would have been a celebrated asset for the still-unified conclave of Lawful deities, good and evil. Jazirian would be reduced to one voice out of many in the throng, making accusations that... what? Asmodeus wasn't nice? Say what you will about him, he's Lawful to a tee.
While Ahriman's body bleeds out in a barren and terrible corner of the multiverse, Asmodeus puts a plan into motion to seize total authority over that plane, recovering his true form and protecting it from the interference of Jazirian or the other gods. Binding himself to the powers of Law, he secures safe purchase of his living tomb by leading the forces of Law against the demons, ensuring they never strike as far as Hell. In negotiating the execution of his duties apart from the godly realms, he strikes a deal that serves him with a source of power from which to regain strength over the eons and lets him undermine Jazirian and the other gods by stealing their worship. Ultimately, by choosing to become something weaker, a mere shade of his own magnificence, Ahriman becomes Asmodeus, a wholly unique entity with a much stronger position compared to the other Serpent of Law.
Will that work for you?
As you have said you are willing to consider Spelljammer rules, I can pull a few quotes in for you that might help clear this up.
From Spelljammer Core Rules: Book 2, Concordance of Arcane Space, p. 17
Clerics in Space: A cleric may not regain spells above 2nd level while in the phlogiston. This is because he remains out of direct
contact with his deity.
A cleric who enters a foreign crystal shell is similarly restricted.
The only exceptions to this occur when a cleric's deity (or a similar
power) is recognized in the foreign shell or the cleric casts a
successful gate spell, affording his deity access to the foreign
shell.
Any god or power is considered "recognized" within a sphere if he has
worshipers and/or an organized church anywhere within that system.
This does not mean that a PC cleric can move into a shell and
automatically establish a base of worship for his god. For a deity to
be a recognized power in any shell, he has to have an established
base of worshipers somewhere in the system.
Additionally, the same rulebook adds the following spells on page 22
Contact Home Power (2nd level Conjuration)
The user of this spell establishes a tenuous link through the Astral
Plane between his present location and that of the power he venerates.
This link permits the priest to regain his spells as if the god is
recognized and venerated within the sphere.
The duration of the spell is one week, as long as the cleric remains
in the same crystal sphere in which he cast the spell. Passage into
another plane (even the ethereal) will break the spell.
and
Detect Powers (2nd level Divination)
[This spell] allows a cleric to determine if there are friendly gods
and/or other powers within a crystal sphere so that he may recharge
his higher-level spells within its borders. It also reveals if the
god(s) revered by the cleric are worshiped within that sphere.
In this case 'friendly' is defined as 'same basic attributes and portfolio' or possibly 'same god by a different name.' As two examples...
-Paladine and Bahamut are the same god, but known by different names and roles in two different spheres.
-Reorx and Moradin are two distinct gods (from Krynn and FR respectively), but they are sufficiently similar that a cleric of one could receive spells from the other.
So, if you take these pieces of information into account, it gives you the following answer:
When you are on a different world within the Prime Material Plane, you may only regain spells up to 2nd Level unless
1: Your god has an established base of worshipers on that world, or on another world within the same Sphere. This implies that the other gods in the system don't actually get a direct say in the matter...though they could certainly stamp out 'heretical religions' if necessary.
2: You use Detect Powers to locate a friendly deity (or your own) and refresh your spells via them.
3: You use Contact Home Power on a weekly basis to 'phone home' and keep that connection open so you may reload your spells.
4: You use Gate to connect directly to your deity's domain and refresh your spells from them directly.
Now, I do not have access to the Planescape books, so I am not 100% certain that this isn't contradicted somewhere in those books...but if you are willing to abide by Spelljammer rules, that's how it works.
Best Answer
Celestial Forces
Solar – much too powerful for a pit fiend
Of celestial creatures, the solar is the most powerful. In fact, solars considerably more powerful than a pit fiend. They are CR 23, to the pit fiend’s 20, but more importantly, the solar casts divine spells as a 20th-level cleric. The pit fiend merely gets a number of spell-like abilities – powerful ones, no doubt, but the the sheer magnitude of power that true spellcasting can bring to bear is simply overwhelming. Worse, the solar is too powerful to be affected by the pit fiend’s most powerful non-wish ability – blasphemy. And the pit fiend is not powerful enough to resist a holy word as cast by the solar, which will paralyze, daze, and weaken the pit fiend. The power afforded by wish, of course, is immense, but pit fiends get that only once per year – solars get it once per day.
Planetar – if prepared, can still guarantee victory over a pit fiend
Planetars, a step down in the angelic hierarchy, are perhaps a more reasonable match, though 17th-level cleric spellcasting is still incredibly potent. With a single boost to its caster level, their holy word could daze a pit fiend – though the pit fiend’s blasphemy will do far worse to the planetar. Still, cleric spellcasting is flexible, in ways that the pit fiend’s spell-like abilities are not. The planetar can use those spells to protect himself, ensure he will act first, and so on. A 17th-level cleric can kill a pit fiend, therefore a planetar can.
But it will take very careful leveraging of that 17th-level cleric spellcasting, and the planetar is likely dead if the pit fiend ever gets to act freely, even once; the planetar has only 14 HD, and the pit fiend can get through their 133 HP very quickly if allowed to. One blasphemy is all it would take. The planetar can only win if he is specifically prepared for a pit fiend hunt that day. If a pit fiend simply catches a planetar on the field of battle, the planetar is almost-certainly dead.
Astral Deva – not a prayer
Astral devas, unlike other angels, do not have cleric spellcasting. As such, they lack options that they can attempt to use in response to a pit fiend’s might. A sole astral deva is dead against a pit fiend.
Trumpet Archon or Ghaele – maybe a chance
Trumpet archons, on the other hand, do get cleric spellcasting – 14th-level cleric spellcasting. This puts the trumpet archon at a huge disadvantage against a pit fiend, but cleric spellcasting really is just that good – a very careful, very prepared trumpet archon might be able to take a pit fiend.
This winds up being very similar to the planetar, but with the odds that much further stacked against the celestial, since the trumpet archon lacks 8th- and 9th-level spells.
A ghaele also has 14th-level cleric spellcasting. They’re also even less powerful, in all other ways, than is a trumpet archon. Still, since both are very easily killed by a pit fiend in a single round, their odds are probably pretty similar – that is, almost zero, barring shenanigans with that cleric spellcasting.
Fiendish Forces
Balor – considerably more powerful than pit fiend
Balors are the demonic counterpart of the pit fiend, but like the solar, balors are much more powerful than the pit fiend. Their blasphemy is powerful enough to affect solars, and high-level dominate monster means they can bring in some ridiculous allies. Solars are still considerably stronger, but balors would, at least, give them a fairly hard time.
If a pit fiend leads the forces of Hell, there either must not be any balors available, or some very specific plot explanation would be necessary to explain how a pit fiend manages to push around balors.
Pit Fiend – Mechanically inferior
As indicated above, solars are massively more powerful than pit fiends, and planetars, a step down from solar, could guarantee victory against a pit fiend if sufficiently prepared. For that matter, with enough shenanigans, so too could a trumpet archon or ghaele. On their own side, balors are dramatically more powerful. For a pit fiend to stand up as the leader of Hell, there has to be more to things than their stats. On their stats alone, they simply are not going to win.
The default cosmology gets around this by having pit fiends be smart, ruthless, and organized. They can handle balors, because demons are almost-always solo acts, while the pit fiend has legions behind him. They can beat angels, because angels have scruples, and the pit fiend does not.
In short, pit fiends survive, and even thrive, because pit fiends know all of this. They know they are weaker, in straight combat, than the angels. They also know they’re not going to stand toe-to-toe with a balor and survive. Of the three, they have the fewest innate advantages. And know it.
Therefore, they simply aren’t going to get into that fight. The terrible strength of Hell is in their plans and schemes and betrayals. The pit fiend won’t fight an angel; they’ll corrupt someone. Someone close to the angel, or that the angel will rely on in a crucial moment – and then they will turn on that angel. The pit fiends each have vast forces of devils at their command; the angels are much more the servants of good gods, with only that individual gods’ forces, whatever they are, potentially available to them. The pit fiend will likely get what he wants, but it won’t be through straight combat. That would be much too predictable and much too stupid.
So for your game, assuming you aren’t simply eliminating the balor, solar, and most likely the planetar as well, the pit fiend needs an extra edge. Some reason why he can win, even though the stats say he loses. I recommend the default explanation, personally; make a balor the Dragon to the pit fiend’s BBEG, too proud, quite possibly too crazed, to be an effective leader, and disinterested in the logistics. Make the angels have too many vulnerabilities, too many things they are trying to protect, and too many things they simply will not do, to force the might of solars to be spread too thin to just tromp the forces of hell.