How about any creature of Large size or smaller for Alternate Form?
Is that IN ADDITION TO or INSTEAD OF the ability to become a Huge Animal or Humanoid that's already in Alternate Form? I quite like the idea of a Dragon Ruler being able to disguise themselves as an Elephant or Storm Giant.
Bahamut-style immunities will work.
For clarification, I assumed you meant I should include Bahamut's immunity to acid, cold, electricity and fire to the suite of immunities Pearl already possesses.
Let me know if you'd like me to remove them.