I try to group with fairly mixed ability-- for the most part. If I have a couple of kids that need so much help the other kids won't be able to work independently with me there all the time, I will group them together. Likewise, sometimes I group my gifted kids together since they spend so much time helping other people.
I change tables every 3 weeks. I try to balance it so everyone sits with everyone. It's not fair for the calm kids or the strong academic kids to always be paired with noisy kids or the kids who ask them for help all the time- however, mixing is good for the most part. I think a break once in awhile is good too.... this is pretty ambiguous... sorry!