I have a bristlenose in my SA tank. She gets after it and is very active. She also doesn't take any "crap" from any of the inhabitants.
You could also put a common pleco in, but you will have to take him back or relocate him in a few years. I have seen some at my LFS that give me nightmares, they are huge!
IMO the more expensive gold nuggets are awesome, but I would hate to lose one due to being stressed out in an aggressive tank.
Good luck, let me know what you decide. I have a 55 gall CA tank that I was thinking about adding a bottom feeder to as well.
Does anyone know if an albino african frog would do well in a 55 gallon cichlid setup?