The obvious choice would be Quinn as Regina, Brittany as Karen and Satan as Gretchen but if I wanted to get more global, I think Kitty would be a cool Regina, Lily could be Gretchen and Hugo could be Karen.
Santana’s not really a Gretchen, is she? Gretchen’s kind of the nicer one who goes crazy that Regina gets all the attention. That seems way more like Quinn than Santana to me… But anyway, since you skipped the ‘and the rest’… I could see somebody like Marley being Cady? She’s this nice, sweet girl who lets people steer her in the wrong direction. And I can totally imagine Rose and Piers as Janice and um…the guy who’s too gay to function? Not because they’re gay but because they’re friends who I could see being awesome masterminds if they wanted to be.