I actually like the idea of Galecki for Butters quite a bit. He's likable, dorky, and has great comedic timing. By the later books, he'd have a chance to showcase a lot more dramatic acting, too, so he'd have some range to worth with.
Billy the Werewolf, and all other werewolves in his group, are young when they first show up in the series. If any type of adaptation goes forward, they should certainly be cast with actors no older than early 20's.