I have the reverse story. I got a baby motor and put it in a 65g with a bunch of tetra and barbs all what I call flitter fish and they nipped at the ray over a day or so and took out its eyes and turned it's disk ragged. Saddly, for them I netted them all and took them for a swim with my big fish in my 300g and they lasted oh say 15 seconds.
i've been debating putting my 7 inch ID shark in with my rays, at least on my female retics side of the partition as she's much more docile, but then i wonder what would happen.