I don't think your options are limited to the Tanganyikan shell dwellers,
but doing the Mbuna from Lake Malawi, IMO, wouldn't be good for your tank.
You may want to look into a pair of Julidochromis regani from the "Kipili"
area of Lake Tanganyika, as they can be quite colorful. You may also
consider a trio of Altolamprologus calvus or compressiceps also from Lake
Tanganyika. I'm pretty sure you'll get responses saying Labids, P. saulosi,
P. demasoni, Cynotilapia or other small Mbuna will do fine, and you'll
probably hear some testimonies to the fact. IMO, the tank is too small for
Mbuna.
The shell dwellers from Lake Tanganyika are very interesting to watch
interact and can make for a great tank. Just a thought. =)
Tim
www.fishaholics.org