Rainbows are open-water fish, better suited to a longer tank, more swimming room. Tetras or rasboras would be better, or a school of tiger barbs, though they tend to be fin-nippers, and gourami's would be tempting targets. Rosy barbs are peaceful fish, hex tanks aren't a good design for territorial species in general.
If you want to keep African cichlids, a 4' tank is a fair size for some varieties. Yellow lab's aren't very aggressive, relatively speaking. Avoid the auratus and kenyi though, they are holy terrors unless you have a much larger tank.