I would avoid Auratus or Kenyi if you're looking for a peaceful tank. 15 Mbuna in a 4 ft tank sounds right. As Coler has said, it's best to overcrowd them or you will often times end up with one fish. I would always do groups of 3 or more for any species. Pairs don't seem to work so well due to male aggression.
Pittbull's stocklist would give you a good mix to fill out the tank.
Pittbull's stocklist would give you a good mix to fill out the tank.