Probably just a yoyo loach. I'd gotten a small school of three, and a couple weeks later picked up another to add to the school. Problems started in that he was larger and outgrew them quickly, then began bullying the other three. For the past few months it hasn't been too bad, but I found one of the smaller yoyos on the floor today apparently chased to the point of jumping out, so he got pulled out and is in a 10g QT until he can go back tomorrow. (also got to learn their spines can hurt a bit...)
I've had a school of CAEs I picked up when a LFS pointed me at them in place of proper SAEs. They hadn't started any problems, but were approaching 4" and the RTS was avoiding them rather than chase them, so they went back.
Some of the fish seem to be particular to the one you get rather than the species. The RTS only starts chasing the others when I drop in frozen brine shrimp or the like. I also have a Florida Flagfish in the tank now who seems fine, but just runs around with the mollies, doesn't bother the molly fry, and doesn't care for the beard algae.