Hmm, well, I don't think I'd keep red tailed black and rainbow sharks together. They're so similar to each other and both are bottom dwellers so are probably likely to fight unless the tank is big enough so that it allows them to occupy different areas.
Bala sharks and either RTB or rainbow might be ok though. I don't know for sure - I've never actually seen anyone talk about this before but balas aren't in the same genus as RTB or rainbow and they also don't occupy the same level of the tank. They dwell in the mid range of the tank and are social with their own species, unlike RTB and rainbow. So that points to them not being territorial with other 'shark-like' fish.