Unusual casting

I'm guessing it was because they couldn't (or didn't bother to) obtain permission from the real mouseketeers to portray them. So they made some generic ones instead (although a few were clearly based on the real ones).

I wondered same thing. "Moochie" appeared in the Spin and Marty serial of the Mouseketeers but he was never a Mousekeer. However, he was a Mouseketeer in the movie about Annette.