Mbuna's don't exactly 'pair up'; they're polygamous ('harem') breeders. An ideal breeding group would be comprised of 1 male and several females (2-4) per species. A numerical mbuna "pair" could spell disaster for the female (over harassed before and after the spawning). Multiple females divide the male's attention among the group.
For the most color (variation-wise) you might want to go with a dimorphic species (matured males and females being differently colored or pattered), particularly the species where the females are also colorful (such as pseudotropheus saulosi, metriaclima kenyi, melanochromis johanni, etc).