I would recommend Apistogramma borellii, or A. trifasciata. Both these species are a tad smaller in size than A. cacatuoides, as well as adaptable. A. borellii is also quite peaceful as far as most cichlids go. I especially love the color variety "Red Mask". I find my A. borellii to be more outgoing, easily greeting me when I visit the tank. A. trifasciata tends to be more shy. If you create a complex, dynamic aquascape this will help them have their own hidey spots.
Most of my Apistos are kept in 15 gallon tanks as trios - although 15g regular (24" x 12") not a tall . I've even kept and breed several in a 10 gallon, although you need to keep a closer eye and have a back up tank. (Sometimes the females can become too protective when guarding fry.) The only Apisto I've had exceptional aggression problems with has been A. baenschi (sp. Inka) - I do not keep these in anything less than 20g long.
Be careful with the Laetacara sp., although many sources label them as being quite peaceful, I've found the opposite true. Quite often LFS may label them as Curviceps but more often they are L. dorsigera. While they do not bother the other fish, once they started pairing up to spawn they are very vicious with each other. Often inflicting deep, deadly, wounds. They have a powerful bite.