Castexies are known to be a little more agressive then other rays, i dont know if this is just in the breeding periode or in general.. I have kept many small rays with big rays, but it is a hassle(atleast for me as i have 9 rays).. The food _ALWAYS_ have to be small enough for the smallest one to eat.. So there is serious cutting going on at feeding time.. All the food has to fit the smallest ray, if not, they wont get anything.. They will take a big bite, work on it til they give up, and by then, everything else is eating. Bin there, seen that