Depending on the temperament of the red tailed shark, they could be bullied to death or totally ignored. I'd take the benefit of the doubt and not put any cories with them. a pleco might be okay.
I've got a RTS is with cories, a small plec, mollies and tetras with no troubles at all. But like onidrase said, it all depends on the shark, maybe I got lucky. Also I've had it from very small so it has grown up with this from the start.
I would only consider putting a RTBS in a community with other riverine fish that do not try to occupy the tank floor, for instance Giant Danios or Filament Barbs in a 5-foot tank.