Loaded question...
Well, I would first not get a RTS. They are the ones who generally cause problems. There are certainly individuals that do fine in a community setting but most of the time, things don't end up working out. If you become set on one, just keep your eyes peeled for any aggression and make sure they can establish a territory of their own.
Even with a rainbow shark you would want something robust enough. A larger gourami species (like blue/gold/opaline/3 spot) would do fine as would larger barbs (off hand..odessa, rosy, etc). "Larger" loaches like yoyos or zebras could also be a possibility.