i too have noticed that my Male leo will is always picking on my motoro, or the Hybrids. They other rays all seem to get along fine. The leo is the smallest but by far the most aggressive.
How does your leos aggression compare to your castexi? I have yet to own a black ray but my jags are the most aggressive I have owned. Flowers like to bite but they do not take return abuse very well.
My castexi are aggressive eaters but don't nip at other rays really. All my rays at feeding may give a little nip or 2 occasionally but the leos do it all the time even when not feeding.