There are smaller africans, you're looking at 20 gallons for a lot of the Neolamprologus shellies, as well as the Julidochromis genus. That's about it for small Tanganyikans.
As far as Malawians...the only fish I can think of that you could do in a smaller (30 gallons+) tank would be dwarf mbunas (Labidochromis).
Hope this helps,
Josh