I've been researching Tropheus for some time (haven't yet pulled the trigger on them, maybe some day), and it is my understanding that a 55 gallon would be a bare minimum setup for a group- as gecko says, 12 or more to keep them from killing each other, and I've heard to start with even more than that. I wouldn't personally mix them with Mbuna, although I'm sure plenty of people do it and get away with it. Plenty of people also keep them as single specimens, which I wouldn't do because they are so social (albeit, aggressive) with their conspecifics.