as far as africans, lake malawi is sortof divided into two types of fish. there are the haps (which get a foot or more long) and then there are mbuna (rock dwelling). the mbuna dont get very large-most get around 3.5-4 or so inches.
lake tanganyika has smaller fish-i have never kept tangs so i cant give input. but i would say get the 6 foot tank-most africans (haps and mbuna) need a tank that is long, and not square (whereas a square tank is good for angels and discus).
decide which lake you want to go with, and then start researching the species. if you get a 6 foot tank, i would recommend lake malawi cichlids-or peacocks and a few haps.
let us know what species you like, and we can help narrow down your stock list! best of luck to you! Mel