Lei and Will probably wouldn't have interacted too much so far since they're in different years and different houses, although I'm sure that Lei has noticed him given his loudness, haha.
And I don't see any reason why Leicester and Booke would have had any problems, so they'll probably be cool with each other! I'm not really sure how much they have/would interact, but who knows? :)
So Lala is probably the character of yours that Leicester would have interacted with the most, just cause she's also Slytherin. Hmmm. Well, Lei's not really big on the blood/house discrimination either (he's got friends just about everywhere, I think, hah), so they have that in common. He would like that she isn't afraid to say what she thinks. >.> I'm not sure about much more than that, if they would know each other more than just acquaintances.
It's reeeally possible that Leicester and Viola know each other because they're both from pureblood families. But it seems most likely that they haven't really interacted so much in Hogwarts itself, but I think there's a definite possibility of them interacting in the future! And haha, yeah, picking on/teasing is likely to happen if they do or have interacted very much. :P
I also don't know how much Leicester and Aidan would interact, although they are in the same year. >.> I don't know! But Lei really wants to try this Jötunheimr Brandewijn one day and would probably enjoy hearing about where Aidan comes from. And he would be pretty amused by how he makes sure people know he's not French and stuff. :D