i have a few yellow labs and they are extremely calm for your tank i would recomend getting about 8 and have 1 male to 3 females with that combination you might even get some fry good luck
Including fry I've got 6 yellow labs in my 65g mallawi tank. The yellow labs rep is that it is a peaceful mbuna but I've got the exception to the rule as the alpha male rules the tank with other supposedly "aggresive" species of mbuna. The more the merrier as the aggresion amongst the yellow labs is dispersed over all the fishies. My labs breed like crazy so it shouldn't be a problem getting fry inside a few months.