The big problem that I had wasn't really the technical end of what they can/can't use in each form, my players were looking to use Multiform as an excuse to min/max. For example. When they are out of hero ID, they wanted a 380cp (Billy Batson) and a additional 380cp Shazam vs a 400cp Superman/Clark Kent. I think the thought process was "I'll spend the 1/2 phase to change if I need a skill then change back to my uber form". When the one player brought that up, the other players wanted to work that into their character concepts just for the extra CP. That's when one of my players said "Is there a reason not to take Multiform if you have a secret ID?" I hope that made sense lol