55 gal African Cichlid stocking thread

First red flag, 3 demasoni; less than 10 (perferably 12-15) will quickly become 1. They need to be kept in large numbers to spread the conspecific aggression around enough. If you really love the demasoni a big group with 5-6 yellow Lab's would work in a 55.
Personally I'd stock yellow Lab's, rusties, and maingano's for diversity of color/patterns and general long-term compatability.

IME acei play a bit rough for a 55, though that may depend on the collection point/species variant in question. I'd drop the johanni, yellow females and blue males are sort of redundant if you're stocking yellow Lab's and maingano's. Not to mention that a pair of ANY mbuna is generally a bad idea...
Sexfasciatus are Tanganykian, and aside from the different dietary requirements than mbuna really need a larger tank, preferably of their own unless it's a really big tank.

Have you considered Red Zebra or Cobalt blues? Very bright and vivid colors, readily available, and not to aggressive IME

For a 55 gallon I would pick 3 species and stick with that. Acei, yellow labs and rusties are all more mellow mbuna and a good choice for beginners. Acei get a bit large so keep that in mind. I would also avoid maingano. They look very nice but most everyone I know has had to eventually get rid of theirs due to aggression (me included). Zebras types are nice but can also be terrors, kind of depends on the individual.

I think blue2fyre is giving some pretty solid advice. I probably wouldn't go more than 3 species of Malawi Africans in a 55 gallon tank because of space limitations. The limitation being the footprint of the tank providing enough space for the fish to establish territories. Acei, Yellow Labs, and Rusties are all very good fish to start an African community tank. I realize every "rule" is made to be broken, but if you want as painless an experience as possible it is good to follow a few of them.

Recently I decided I wanted to "break" one of the rules and I tried putting some Z-Rocks in a mbuna tank. My wife is still chastizing me for being cruel to the little guys ...