A ray will usually only consume what they can shove in their mouth and put in their 'crushing' grinding type teeth..
However, they are also known to leap on broad-bodied fish at night and chomp on eyeballs (discus)..
The eel would be like a big tastey worm for him to chomp on as soon as he could fit it in his mouth..
Aros should be fine..
From what I have seen from the bay snooks is they are passive predatory, meaning they will only eat things that fit in their mouth.. I could be wrong, but it seems as if they are very similiar to datnoids in disposition and shouldn't bother the rays..
Ray growth depends alot on feeding and water changing schedules.. I have yet to grow them out to a good size (just imported and resold), but like most fish they have intense growth spurts during juvenile sizes (6-12"), and then start to plateau out as they get bigger. Depends alot on the species too.. A smaller species like a reticulata will grow and plateau sooner than a species like castexi, tiger, or flower which get very large and can be aggressive eaters.
Just keep in mind Rays derived from saltwater fish and their osmoregulatory system works differently and they release massive amounts of waste in comparison to their size.. The smaller tank you plan on starting them in, plan on doing alot of frequent water changes if you are feeding them properly (which is alot)..
Also, sometimes rays don't like to move from tank to tank even if its the same house.. water conductivity could be a little different, and cause stress.. Would be a silly loss, especially if you could of just waited a few months.
Thats why I say if you can be patient, wait to just acclimate them straight to the 350g, and allow them to be some of the first fish in the tank to get settled, and then add tankmates later.
Cycle the 350g with raw shrimp, fishless cycling, to get an enormous amount of bio-load built up.. Add the rays, and monitor water quality closely.. Allowing them to settle in first can work to your advantage in many ways when adding other tankmates later.