Compatibility is a tricky subject. What works for some is utter failure for others. And just because something 'works' does not mean it is the best thing for the fish. Peacocks are not as aggressive as mbunas. They are from deeper down and have different behaviors and diets. The tank setup is different for peacocks and mbunas. Mbunas need a ton of rockwork, either wall to wall from bottom to top, or two or three big piles that take up most of the tank. Peacocks need mostly open water with two or three smaller piles of rockwork. If you do it right it could work, but even then most likely would not work as well as if you only did one or the other. I have seen them together and I now have separate tanks for mine for a reason, they do better separated. The peacocks will be more comfortable and therefore have better colors, better growth, and be much less likely to have any health issues. By mixing them you will probably end up with a peacock or two being bullied by mbunas, and the other peacocks will still be stressed even if they are not getting the worst of it. Even when small the mbunas at work pick on the peacocks if we keep them in the same tank, so we even keep them separated when young at my work.
If you go with mbunas the stocking will be a tricky issue. Overstocking can decrease/spread out aggression. However, High stocking makes it that much more vital that big water changes are done every week, if not semi-weekly. You will have to have a ton of filtration, at least one Fluval FX5 if not two. I would shoot for more like 30-40 but add them as described. Add them in groups by species from the least to the most aggressive.
I would discourage you from mbunas because of the maintenance required due to overstocking, and because even then aggression can be a problem. The peacocks are much less aggressive and are therefore easier to deal with. My 150 has about 20 or so peacocks. The most they do is a male will chase another fish out of his territory. My mbunas are still much smaller and already I see twirls of fish and mad dashes through the tank, much worse than in the peacock tank.
