For an animal that reaches around 14" diameter that will be way to small. They will barely be able to turn around. You want a bigger footprint tank for stingrays to swim around and bury themselves. I have a couple stingrays in my system that are the same size, but my tank is 8ft x 6ft so they have a ton of room to swim naturally. They are alot of fun and voracious eaters! If you are set on keeping them and can't find a tank big enough then look into stock tanks that are alot cheaper.
As far as tank mates just restrict anything that has jaw like teeth and can nip at them. Trigger, Angels etc...