Though it's only in book seven that that will really become problematic. As long as it's merely being mean to Gryffindors, well, they do have that rivalry going on.

Slughorn is pretty much the only one to fit that description...

It's just Rowling not being very good with numbers. It more or less adds up - with the Sorting, the class sizes and all that - to assume an average of 5, perhaps a few more, per gender per year per house. But that means no house is even as big as 100, nevermind 200, and there are a few other places where she suggests too large numbers for the students as well, iirc.