A 20 high is a very small tank, especially in terms of footprint, which is what is most important for territorial fish such as cichlids.
You will probably be OK with just the Julidochromis transcriptus (although I'd only get four, not six), some bottom fish, and some midwater schooling fish. In a slightly larger tank, I would suggest that you get some Cyprichromis leptosoma, a schooling cichlid from Lake Tanganyika, but in this case I'd suggest about (5) Celebes rainbows. Unlike rasboras, which (as ChilDawg noted) like soft, acidic water (the exact opposite of Lake Tanganyika), these fish can tolerate Rift Lake conditions relatively well.
And then I'd add (3) Synodontis petricola. They don't get too large, and will add activity to the bottom of the tank. They can also take care of themselves.
I'd also leave out the bristlenose pleco. Julies like to graze on the algae that grows on the rocks and glass, and the pleco won't leave any for them.
Just as a side note, I'm surprised that no one in your LFS's mentioned the incompatibility of the fish mix that you had selected, or that the A. nyassae were far too large for your tank. You might want to take everything else they tell you with a large grain of salt.
Finally, I'd recommend that you invest in one or more of Ad Konings books about Lake Tanganyika. They contain a lot of useful information about species' behavior and habitats.