Good information here. Travis spot on. I would absolutely discourage a reticulated stingray to anyone but an expert who knows exactly what they are buying. Historically they were always wild caught, infested and infected, and only eating live foods. If you found one now, likely captive bred but they still have small mouths, high metabolisms, and get huge.
For a smaller tank, if you insist on a 6x2, hystrix or proven mini marble are the only options.
Save for a bigger tank and do it the right way. You'll have more stocking options and greater chance at success.
Good luck