I realize by your initial choices, that being geographically correct is not important in your tank. But if you were to choose dithers that come from the area all 3 of your cichlids come from, they would either be live bearers like Gambusia, or Poecilia, or tetras like Astyanax.
Astyanax tend to be very nippy, so in a small tank like the ones you describe, they would be far too aggressive, but some of the larger swordtails or sailfin mollies would work well, and would probably stay near the surface away from the cichlids, providing movement in that area, and the constant dropping of their fry would provide plenty of extra snacks for the cichlids.