i'd say they should be ok together as long as the size difference isn't very big.. i'm currently keeping a 15" disc motoro with my 16" endlicheri with no real problems.. the only minor problem is the motoro took a bite out of the endlicheri's anal tail but nothing too serious though.. the only fish my motoro got real aggressive to is a 8" royal knife which it ate
my bichir is the most active fish in any of my tanks she is very social i dont think that they would bother a stingray just hope your ray doesnt eat them
Since both are bottom dwellers, there will be confrontation. Some people succeed in putting them together while some do not, all depends on you whether you wanna risk it or not...