I'd say all rainbowfish would get on well with a red-tail shark. I have boesmani rainbows (and other species) in with a rainbow shark and they get on great in my 50g. The shark stays towards the bottom and the bows towards the top, so they hardly ever interact.
To keep a small school of boesmani's and a red-tail shark I would say a 50g long would be the minimum tank you would need. Any smaller and aggression (from crowding) could become an issue.