The dimensions are indeed very important when it comes to cichlids.
I have very little experience with tanganyikans so I won't attemp to reccomend anything from those though I'd expect certain julies would work.
However, I can tell you that if you are considering mbuna, the tanks a little small to make a good community set-up. You could try a species tank of some of the more peaceful mbuna though - like yellow labs or p. acei. Remember to over-filter as you'll be crowding the tank as a way of reducing aggression. There's a pinned article about keeping mbuna that might give you some more ideas.
Having said that, as the tank's small, perhaps you would be better off looking at some of the neotropical dwarf cichlids from south america - rams, apistos, keyholes and the like. They are more suitable for this size tank and could live with most non-nippy schooling fish as tankmates.
If you are set on Africans, you could always go for a pair of kribs/pulcher - though I suspect you'd rather not
If you are set on rift lake cichlids, perhaps you'd like some shelldwellers? They are extremely interesting to watch and, the smallest, n. multifasciatus, can be kept with most community fish as well (to fill out the top areas in the tank) because they are too small to cause any damage despite being quite territorial. These particular shellies are also an absolutely endeering species - beautiful - and extremely easy to breed. These probably aren't quite what you were considering but they would work in a tank that has a slightly smaller footprint (most 29s have the same footprint as a 20 and so aren't realy that great for mbuna or tang communities) than ideal and they do have an incredible amount of character for such a tiny fish. Their unusual habits and shell-dwelling behaviour is intruiging and they can be quite captivating little fish. There are, of course, other shellie species as well. Someone posted a lovely picture of a multi a while ago in the member's pics section (so maybe take a look at that) or you could always do a google.com search.
