Yeah. It could work.
But it implies that the character creates all the different forms at the beginning. Fortunately, characters' creation is not too long in Supers.
However, I don't know if this rule could work for a character who's always changing of forms at will (like a genie) and who needs to create a new form on the fly. I think in that case, the GM could authorize the player to chose a power or an aptitude that he needs on the moment.
If I have any more thoughts about that, I'll post them later.
I gotta go.
Sorry for my English, but it's not my primary language.