Oh, but they are on the Quidditch team together. Which doesn't particularly mean they have to like each other. BUT they do have to work together. Also, I'm sure Regulus doesn't hate him. ...Yet. It's possible. I mean, if Pearl tells all!
In any case, I expect at least civility. Being housemates/teammates/etc. Regulus spends an amazing amount of time in the 6th year Slyth boy's dorm.. Not that uhh... anyone particularly knows this... no way at all. :)