1. Yes, I've had my P. elegans and P. interruptus breed in my community mbuna tank.
2. Depends on what type of fish you like... Haps, Peacocks, and Mbuna are all similar but very different as well. Are you looking for open water predators, sand dwellers, rock dwellers??
3. 10-12 fish should be fine in a 55 gallon for quite a while, if not for life.
4. I keep my water at 80F, 8.2 pH, and fairly hard; it comes out of the tap this way for me
5. Mbuna especially get a bad rap for being absolutely nasty. I realize all individuals are different, but I've never had a problem with aggression in my tank... at least so far haha
Feed them a high quality protein based diet, like NLS formulas, for the best growth, color and overall health.
^ Ya if you want them to catch bloat lol, its best to feed them a LOW protein based diet and a quality vegetable flake food. Mbuna are primarily herbivorous. NLS is good though