This is a terrible plan. I, having kept stingrays before can tell you this will not work out. Cortez stingrays will be fine in a 6 foot x 2 foot tank with no rock work for a year or two if they start out as small as a teacup saucer, but they really need the 6 foot x 3 foot tank long term. For stingrays, one shouldn't look at gallonage; that is nonsense. They should look at the foot print and I feel one Cortez ray needs at least a 6x3 foot print (clear of live rock and with a very fine sand bottom) to live out a happy life.
Countless times I see my LFS put stingrays in with puffers/triggers and this is the ultimate no no. Puffers/triggers are very curious/hungry animals and most of the time bite off the stingray's eye and take big chunks out of their discs. Please do not put a stingray in with a puffer/trigger.
As for reefs, I have found great success in keeping coral with elasmobranchs. It is just important to remember that if you do so, it is the elasmobranchs tank not a reef tank. One should have a bigger foot print if the wish to do this and the tank should still be mostly sand not mostly reef. What I did was have coral in the two back corners of the tank and the rest of the tank was oolitic sand. If one tries to keep coral in the tank they should remember not to get sensitive to touching coral (such as the elegance coral) or corals that are known to have a vicious sting (like hammerheads). They also obviously need high powered lights and good filtration (which they need anyway to keep the ray alive).