Adult pair in a 20 high (for breeding) then otherwise a pair per 2 ft of tank e.g. 2 pair in a 75, 3 in a 125, etc. Depends also on temperament.
Cories, raphael or bumblebee catfish, tetras etc are good tankmates. Small neon tetras may get eaten...black neons or cardinals or larger bodied tetras are better. Apistos can work in the bottom but generally one species per tank because of hybridization risk and they attack other species of apistos or similar like golden dwarf cichlids. Hatchetfish can work in the top given a good lid and something like a 55g or 75g.