depends what you want to do Do you want tangs or malawis: with malawis the species are divided into three broad groups based on their behaviour: mbuna which are smaller rock dwellers, haps which are generally larger open water species, or peacocks which are small to mid sized bottom and midwater dwellers. mbuna will require a large amount of rocks while haps and peacocks will require much more open space, though sand is the preferred substrate for all three. I personally like haps but only a few will not outgrow a 55, plus the females are generally drab though they have very interesting behaviour. If you wnat malawi cichlids I'd do a mbuna tank in your case because they are the smallest malawi cichlids so the 12 inch width won't be much of an issue assuming you pick your stock well. they also have the most vibrant colours. However some mbuna are easily among the most aggressive cichlids. All malawi cichlids need to be kept in harems or as individual males otherwise male - male and male - female aggression will result in casulties.
For mbuna you pretty much need to have at least 1/3 of the tank volume consisting of sight breaks like rocks, pipes, etc... because without enough of these territories the dominant males will force the weaker ones to the top, greatly stressing them. Another trick to limit the damage of aggression with mbuna is overstocking, adding more fish then you'd normally expect in a tank that size. This is because unlike SA/CA cichlids which bide their time then suddenly go nuts, african cichlids are fighting 24/7 adding more fish means that no individual will be singled out for too long until they need to chase someone else. if you decide to go this route be careful not to go too overboard and make sure you have very heavy filtration as you are essentially cramming a relatively large number of messy fish into a tank. With your 55 you could probably keep around 3-4 species of lesser to moderate aggressive mbuna in a ratio of 1 male to 3-5 females. When mixing species avoid similar coloured species because they will assume that they are of the same species and the less aggressive species will get shredded by the meaner species. another issue is hybridization, if you don't mind hybrids it's no big deal but selling them can ruin the bloodlines of captive species. you will probably have to buy loads of fish then sell them as mbuna are not dimorphic until adolescence, and some show no dimorphism whatsoever.
haps and peacocks are very similar in care. they are noticably less aggresiive then mbuna with a few exceptions in my experience. Like I said few haps will be good for life in a 55 but if you find one that is they have similar care to peacocks. the main difference in setup with these vs mbuna is to leave much more open space on the bottom. aulonocara are essentially aggressive old world geophagus, which means that they remain in the bottom half of the tank sifting sand. it is best to keep only one peacock species unless they look very different or the males will fight over any females regardless of species as female peacocks are more or less the same drab colour. However an all male tank would mean more species in the 55.
I don't know enough about tanganyikan cichlids to help you much so hopefully someone will chime in
