Black Stars or Black Sex Links as they are more commonly called, are great birds. We love ours. They are a nice heavy bodied bird and lay very well. We have some that love to be held and a few that love to be tickled. We breed our own here each year.
As for them getting alog with your RIR's it will depend on age and how you go about transitioning them together. If they are all still very young, then there should be little problem. If they are all adults then it would be best to transition them together by having them seperated within the pen by a run of chicken wire. let them get used to each other through the wire until they ignore each other and then remove the chicken wire and there should be just some minor pecking order stuff.