Crossbreeding of Tang's is possible, but mainly among different species of the same genus. (Don't mix 2 types of Julies, etc. in the same tank) There have been a few cases of Julies crossbreeding with Lamprologus leleupi's as well.
Unless you want a species tank, stay away from the Neolamprologus brichardi complex. There are numerous species, closely related, but all have one thing in common. When they spawn, the older fry help protect the younger ones. Eventually, they will take over the whole tank, first crowding the other fish into the corners, then eventually killing them, either from aggression or just plain stress.
Otherwise, the featherfins, (Ophthalmotilapia and Cyathopharynx species) Tropheus, Frontosa, and "jumbo" Cyprichromis species come to mind as needing a larger tank then a 55.