I think the next ability I want to get for Thalron is Polymorph or Mythic Polymorph. Something and I was wondering…

1. Can he only turn into things that he has seen, or if there were any other limitations? (other than the natural, or unnatural of the two spells)

2. I was also wondering if he could use this to turn into a smaller version of himself, using probably mythic polymorph?

he can turn into anything even the things he hasn't seen.

I don't see why not!

Thanks for the quick reply! Hmm… A few more questions came up when I was thinking about it, so if he were to turn into another magical creature lets say, a fire dragon, would he then be able to breath fire, and also have the same fire resistance as it would normally?

You would as long as you had the fire breath spell :)

Even in a dragon form, you'd need to substitute for abilities a dragon has (to keep things balanced for everyone). Fire Breath has been suggested already, and I recommend Fire Meld for the fire resistance.