I have some rays with a big aro right now, but when I had my baby I wouldn't really want to risk it.
From my very limited experience the baby arowana's seem to be sissy's... or maybe the ones I got where. My silver never really dominated the tank and the tinfoil barbs that were supposed to be dither fish ended up being too aggressive for it.
For the ray's the bigger the size I am guessing the bigger the chance of them attacking the smaller meal. It would just be a very expensive mistake if one night the rays wanted a midnight snack...