Try some improv, perhaps?:)
Dude, your impersonations are fantastic and spot on!:) I have really no suggestions for improvement, but as my headline says, have you ever tried improvisational acting? It's not only extremely fun, but it will help you to think even quicker and on the fly, making switching up voices even more of a breeze.
A little advice from one voice actor to another.:) Anyway man, keep up the great work. You're hilarious, and your music kicks major ass!