Why would the Anti-Cosmo and the pixies put on Dr. Rip Studwell and Jorgen costumes when they can change shape?
Anti-Cosmo lost his Fuhgiggly gland (the gland that allows Fairies to alter their shape) to Cosmo in exchange for his freedom. There is also no evidence that the Pixies can alter their shapes.
Response: Anti-Cosmo re-gained his shape-changing power in "The Jimmy-Timmy Power Hour 2." Besides, Anti-Cosmo still has his wand to change his shape.