scavenger fish don't really make a big difference. unless your're overfeeding your current fish will pick up most everything, BUT a neat way to add activity to all levels of the tank (but mostly bottom) would be shrimp. They are scavengers and produce very little waste, as compared to many bottom level fish that are huge waste producers.
ghost shrimp will eat anything, food, algae, dead things, and are big enough that your stock shouldn't really cause them any trouble. Amano shrimp are nice, though they are mainly algae eaters (i don't know if they eat debris at all, actually), but they would also be too big to be eaten by that group. Cherry shrimp are very pretty, but shy and usually very small when bought so quick fish may eat them, or at the very least, worry them into hiding.
My cherries weren't real hiders, but they were still very reserved. My ghost shrimp have tons of personality though. Their pincers are useless against fish but if they are happy and comfortable in the tank they will defend their food, they will even climb/swim up the walls of your tank to grab food off the surface.
If you do get ghost shrimp, try to make sure that's what they really are because there is a type of long armed shrimp that is often sold as ghost shrimp, but unlike ghosties, the long arms aren't harmless. they WILL try to eat your fish. check in the inverts section for more info
oh, and ghost shrimp are also often called glass or grass shrimp.
