Geophagus tend to be bottom feeders/scavengers themselves, so I would limit how many more bottom feeders you add to the mix. Any of the medium sized loaches should do fine though (avoid huge loaches like Clowns). MTS work too (don't add on top of the loaches since loaches will decimate them, do either or), but will tend to be predated on. They breed fast enough that they should make it as long as you add a large batch to start with.