Chemistry may not be an issue unless your water is very soft. Since most people seem to suggest not altering anyways, each of your tanks will have the same chemistry. I do prefer to add special salts and buffers, the fish do better that way IME.
Mixing cichlids, lakes, and even continents is purely a matter of preference (and aggression levels, careful selection, etc.). Some people suggest keeping mbunas only with mbunas, peacocks only with peacocks, shell dwellers only with shell dwellers, etc. Some people suggest sticking to only one lake. Some people suggest keeping lake cichlids together and not mixing them with any other cichlids. Some people suggest keeping them separated by continent, and many people have had success keeping cichlids from all over the world together. It is really up to you, but without experience with all these cichlids it is almost impossible for you to determine their aggression levels and therefore compatibility.
IMO that tank is pretty small for cichlids. If you stick with cichlids I would keep the tank pretty specific. For example, have only jack dempseys, only mbunas, only peacocks, only Tanganyikans, etc.