Mbuna to think carefully about before purchasing: Melanochromis Auratus, Melanochromis Chipokae, Metriaclima Lumbordoi a.k.a. Kenyi, any of the Melanochromis or Metriaclima really. They generally can be pretty nasty, some more than others, especially the ones I mentioned specifically. 12-15 is not too many to stock a 55g with, provided there is adequate filtration and rock work and you have a good mix. I have 19 Mbuna in my 55g and the aggression is almost nill. I really prefer Cynotilapia Afra or Pseudotropheus. Give Pseudotropheus sp. Acei a look, they are beautiful and quite "peaceful" for Africans. I try to do 1m-3f ratios. And mixing lakes, although taboo, can be quite easy and rewarding if you pick fish that are similar in diet and aggression. My Lfs currently has a tank with: 3 Altolamprologus Calvus, a Victorian Rock Krib, a Hap sp. 44, a Yellow Lab, a Blue Socolofi, 2 Julidochromis Ornatus, 1 Astatotilapia Burtoni, and 3 Cynotilapia Afra Jalo Reef. All 3 major lakes in one tank. Pretty unique and beautiful. It's all about what you decide on. Personally I prefer Tangs, Altolamprologines specifically.