He may be right, there is a good chance that rosy barbs will be fin nippers, and the quppies tail can be a pretty tempting treat for barbs, but then again it really depends on the personality of the fish. I have heard keeping them in groups helps to diffuse the aggression. I have never kept rosy barbs with "questionable" fish, but have kept tigers(who are also supposed to be fin-nippers) with more peacful fish. Two did just fine, one nipped fins so I took him out and everyone was fine.