
Isil-zha |
I'd allow you to change into a "young" Medusa(just as an example), if that's what you're asking for. But this is my personal take on this. The rules as written do not actually specify it, though the shaman summon abilities suggest that bigger and smaller (than the bestiary) versions of creatures exist.
I think this may very from table to table, and may come with certain drawbacks.
edit: deleted some nonsense

Isil-zha |
I don't see why applying a template to the creature is a problem, you are not applying it to your character when you cast the spell. But there are young monstrous humanoids out there that you could potentially shape into.
How developed their special abilities should be at that point is a different question that isn't covered by the rules and falls into GM fiat.