I suggest you monitor your FRT personality before deciding to add it with the rays
So far I have managed to comm a 4 inch and a 6 inch ray with my FRTS (around 10" & 7")
Yes I keep 2 FRTs together with my rays in the same tank
So far it worked. No fights or disturbance but it is not something I would suggest for you to try....
Btw my FRTs are always well-fed and so far they do not chew on stuff or nip the other fishes
Oh ya remember that enough space in the tank is a huge consideration.
FRTs tend to be nastier if the space is not large enough for them.