The question becomes, I think, "is the improved access balanced out by the limited hit die and limited number of forms known?" And that is a question, I think.
being able to be a dragon at 11th level, or vermin at 6th Is a bit much I think, since that PrC is basically beast master in 10 levels. And beast form is a very strong ability, is it as good as wildshape, No but then again wildhsape is the most powerful ability ever.
Mmm. Perhaps.I thing the class is very solid, the only changes I would make are the action required to change shape I think you start the lower action a bit soon instead of 4th, 8th, 12th, and 16th. I think id go 6th, 10th, 14, 18th.
Magical beasts and colossal forms aren't enough?also. you need a capstone not sure what you could put there but I believe that any class that don't get 6th level spells needs a capstone
Actually, I was thinking about changing it to vermin at 6th, magical beasts with 1 or 2 intelligence at 12th, and all magical beasts at 18th. Which would leave me hunting for a capstone again, I suppose. Hmm...
(I might also throw in (Ex) special qualities at some point here).