Short answer, no.
I had this question many years ago and still carry around the printout from the 3.5 FAQ (http://archive.wizards.com/default.asp?x=dnd/er/20070731a).
From p76
Many animated objects have hardness scores. What affect, if any, will an animated object’s hardness have on spells used against the animated object? For example, an animated wooden table would have hardness 5, right? How would that hardness affect spells such as fireball, lightning bolt, Melf’s acid arrow, ray of frost, and magic missile?
If the spell in question has an energy descriptor, hardness affects the attack as noted in the rules for damaging inanimate objects (see page 165 in the PH); here’s a summary:
• Hardness applies to acid and sonic attacks. These attacks deal normal damage both to creatures and to objects, and thus would deal normal damage to an animated object (less the effect of the hardness). You would subtract 5 points for hardness from whatever damage a Melf’s acid arrow spell deals to the animated table in your example.
• ...
It goes on to describe every type in the bolded Q there as well.
Yes it is more or less appropriate challenge, though it should be something like "boss"/"mini-boss".
As TheDarkWanderer pointed out Challenge Rating and average party level shouldn't necessarily match, so your party should probably be ok with a creature like that. After all CR system doesn't describe challenge really accurately, as it depends on multitude of different things like pre-scouting and builds.
You may still want to adjust this dragon for it to suit your needs more, but here is estimated 3.5e statblock based on information you've given:
Size/Type: Gargantuan Dragon (Fire)
Hit Dice: 22d12 +132 (275 hp) x3
Initiative: +0
Speed: 40 ft. (8 squares), fly 100 (clumsy)
Armor Class: 40 (-4 size, +44 natural [decreses slightly in later forms]), touch 6, flat-footed 40
Base Attack/Grapple: +22/+45
Attack: Byte +29 melee (4d6+11)
Full Attack: Byte +29 melee (4d6+11) and 2 claws +27 melee (2d8+5) and 2 wings +27 melee (2d6+5) and tail slap +27 melee (2d8+16)
Space/Reach: 20 ft./15 ft. (20 ft. with byte and tail)
Special Attacks: Crush, Tail Sweep, Lava Bomb, Earthquake Stomp, Shed Embers, Lava Vomit
Special Qualities: Darkvision 60 ft., Low-light vision, Dragon traits, Vulnerability to Cold, Immunity to Fire, DR 10/magic, SR 26, Paragon Fortitude, Paragon Fury, Blazing Form, Hardening Skin
Saves: Fort +19, Ref +13, Will +16
Abilities: Str 32, Dex 10, Con 22, Int 15, Wis 16, Cha 18
Skills: whatever
Feats: Multiattack, Power Attack, Improved Bull Rush, Awesome Blow (and 4 more feats of DM’s choice)
Challenge Rating: ~18
Crush (Ex) (4d6+16) DC 32;
Tail Sweep (Ex) (2d6+16) DC 32
Read
this about dragon specials. Note, while it is generally suggested to base these attacks on Con, I personally suggest to base them (as well as Earthquake Stomp) on Str instead, as they are basically area melee attacks.
Earthquake Stomp (Ex) (~2d6+16) DC 32
It is hard to convert this into 3.5 safely, since 3.5 is much less cartoon-styled, so just stomp would hardly be capable of causing Sonic (or what alternative is appropriate for Thunder?) damage. You may want to completely rework it.
Blazing Form (Ex) (~ 3d6)
Lava Bomb (Su) (~ 7d10) DC 27; Shed Embers (Su) (~ 9d10) DC 27; Lava Vomit (Su) (~ 12d10) DC 27
These are based on appropriate Breath Weapons of similar dragons.
I’ve added Fire subtype here, making Vulnerability to Cold and Immunity to Fire constant. You may of course reverse this, but it is how 3.5 generally deals with such cases, I believe.
Damage and Reach are now set as appropriate for gargantuan dragons. I added longer reach for the tail as it seems to have longest reach in the source. Gargantuan dragons usually also have Wing attacks (as well as Crush and Tail Sweep specs). Feel free to remove them if needed.
Bear in mind that this creature will became MUCH stronger when it will lose two of it’s pools. So it isn’t CR 18 from the start, but maybe somewhere close at it’s third pool.
Best Answer
the SRD monster description says "Fire attacks cannot prevent a pyrohydra’s stump from growing new heads (since a pyrohydra has immunity to fire), but 5 points of cold damage does." By extension, creature immunity to damage type means that stumps cannot be prevented from growing via that damage type. The pyrohydra's stump immunity to fire is a natural result of the pyrohydra's overall immunity to fire, and thus one might expect a similar result from immunity to cold.
Worth noting, this means that the only way to keep stumps from regrowing is acid damage. I wouldn't throw this monster at a party unless I knew they had a fair amount of access to acid damage (or such was provided on site).