In answer to your question, no. You are already way overstocked as you are.
That pic is a motoro AFAIK, but they reach 2.5' in diameter. If it is only 12" after 8 years, it must be stunted by being in a tank too small for it. Even the smallest stingray, the teacup stingray, reaches 18".
As motoros get 2.5' wide, I would not keep them in anything narrower than a 3' wide tank (preferably 4'), and preferably 8' long. Best is a large pond for stingrays. Not only will they really love the extra space, they are one of few fish that look better from the top than from the side in an aquarium.