[RPG] Am I allowed to know spells I can’t cast yet

It's still the same in Pathfinder, as described in the "Arcane" section of the "Magic" chapter^[PFSRD]:

Spell Slots: The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his due still gets the slots but must fill them with spells of lower levels.

Emphasis added

However, this doesn't mean the spell actually has the higher spell level. This means that even if you cast Magic Missile from a 4th level slot it would still count as a 1st level spell for all intents and purposes (e.g. being blocked by a Lesser Globe of Invulnerability or calculating its Save DC). If you want to have the spell being treated as a higher level you have to use Heighten Spell^[PFSRD].

Common wisdom says that direct-damage blasting is a suboptimal route for high-tier casters. Damage that doesn’t kill leaves an enemy at full capacity for hurting you or your allies, plenty of low-tier classes can only deal damage, and tend to have an easier time dealing it than a caster does, most direct-damage spells are pretty poor, and so on. There are a lot of reasons to avoid blasting altogether as a caster.

However, it’s worth noting that, at the high end, sorcerer blasting is pretty terrifying and extremely reliable, even by the standards of high-tier, high-optimization builds (see the Mailman). Immunity to HP damage or death thereby is extremely rare, after all, and immunity to being dead just doesn’t exist. And the sorcerer is well-placed to do it really well, what with access to the Sor/Wiz list plus arcane spellsurge. At this point, you’re not handing out damage, you’re handing out death.

So good sorcerer blasting is very all-or-nothing: either you kill, or your blasting was a bad idea in the first place. Much better to apply some very-debilitating condition than to deal a bunch of damage that won’t actually kill. Even if an ally mops up the remainder before the enemy gets to go, that’s an entire turn the ally didn’t spend dealing with a different threat.

Now, if you do want to go the blasting route, do you want to know a bunch of attack spells? No. Knowing any at all basically means you’re “specializing” in it, relative to most sorcerers. You build your blasting on one or two key, excellent spells, not on knowing a bunch of different blasts. That would be pointless and redundant.

Because that is something that is always true of every sorcerer: you do not want redundant spells. You get an extremely limited number of spells known. Your total spells known is, in fact, very similar to the spells a wizard gets to prepare every day, but where he’s only got to worry about his spells being useful for the next 24 hours or so, you have to make sure they’re pulling their weight for the rest of your life.

So no, under no circumstances do you want a “lot” of blasting spells. You may want a “few” blasting spells (like literally 1-3), but even if you specialize in blasting, you still shouldn’t know too many direct-damage spells; a few well-chosen spells should be quite sufficient. You need the remaining spells known to ensure your mobility, battlefield control, versatility, defenses, and so forth. And if you’ve chosen your blasting spells well, there should be no reason to use a different one.

Personally, I’d probably get some lesser orb at low level, swap it out at 8th or 6th (or the last even Sorcerer level I took), get a real orb at 8th, and only get another once I hit the orb’s Caster Level cap. That is, if I was actively trying to be a blaster. If not, I’d be fairly likely to not bother with any at all.