Yeah, it seems as though Mal and Keegan have a lot in common. Both are party boys, both like the girls and both are no stranger to getting into hot water. (Plus those things you mentioned of course). I think it would be fair to say they would be party buddies. Oh yeah, don't forget, they would know each other from Quodpot.

I think they would know each other because of the shared classes and they might have a unique relationship because Quinn is a lesbian. Mal could enjoy just being himself and not worry about the guy/girl stuff with her.

Mal would find Sibyl interesting because she's cute and she plays Quidditch. He loves sports and follows Quidditch, even though he doesn't play it.

Yeah, not sure about Casey. If he ever saw him in the common room or anything, Mal would be nice to him.