If the water is soft (hasn't got many minerals) and has a GH below 100ppm, you can try bigger species of tetras that are not renown fin nippers, rasboras, soft water dwarf cichlids like Apistogrammas, Pelvichachroms and Nanochromis. You can also keep the catfish listed in the next section.
If the water is soft to medium hard (100-250ppm) you can look at some of the peaceful barbs like Ruby, cherry, checkered and golden Barbs. Although cherry and checkered might be a bit small depending on the size of the angels. You can also keep most small catfish like Corydoras, Brochis, Bristlenose, Peckoltias, Whiptails (Loricaria sp) and Twig catfish (Farlowella sp).
Medium to hard water (250ppm +) and you can keep mollies, swordtails, platies and guppies.
Most species of rainbowfish will be fine too and will be fine in water with a GH between 150-400ppm.
There's more info on rainbowfish at the following link.
http://rainbowfish.angfaqld.org.au/Melano.htm