I think a gourami would be fine. The gourami likely wouldn't bother the barbs as they're too fast and if you get a large enough school of barbs, they will just keep to themselves and not bother the gourami. The gourami may set up territory somewhere, but would most likely not be an issue with a semi-aggressive stock. Something on the bottom would be fine too, cories or some type of loach would be good. Clowns might be too big as they like schools and get very large, so you'd need more than one.