Okay, so it seems the rules disagree with themselves in regards to what you can polymorph into, and if your type changes, so for now, I'd go with the actual Spell, which says that you can become a creature of the same type, and that your type does change when you polymorph.
So... can we get some agreements in regards to whether or not Dismissal would actually send a polymorphed creature back to it's "home plane" (or natural plane for a creature of that type)? Since almost no one except for the first few posters have commented on that.
@
Empirate : I am very much debating this. Again, my group had NEVER really optimized like this before, or even used spells to such efficieny... or cheesiness, if you will, so I felt no need to get rid of spells like this. But... if the player is intent on cheesing it out, I may just have to get rid of Polymorph and associated spells. Or find a way to reasonably rework them.