i think so, but i seriously doubt that you'd want to add a freshwater stingray to your "cichlid pond". stingrays are notorious for keeling over at the slightest contaminant in their water; i'd think that an open-air outdoor pond would be a bit risky.
also, the smallest stingray available still has an adult disc size of about 18". if you'll search for stingrays in oddballs, you'll find several threads asking about tank sizes appropriate for rays. i'm not certain, but i think that 6'x2'x2' is generally listed as "maximum smallest for the smallest species" with a 3' wide tank being preferable.
umm, no, i wouldnt mix them. Stingrays are fragile and expensive and an oscar may be too aggressive for it. Besides, in an open air pond, would you really want to risk a stingray with all the potential predators about?