The flowerhorn may try something with the sting ray and the ray would feel threaten and barb it I heard someone had a dovii with a ray and the dovii bit it and the ray barbed it threw the head killed instantly
I guess I'll save myself the trouble from posting and thread and just post here...Which is really ironic lol. I purchased a retic ray (teacup) about 5" in disk size from a LFS around my area. Just put it in my 90g with my flowerhorn and he's been in there for 72 hours now and the flowerhorn pays no mind to him.
Update:
I've had my teacup ray for well over a week and there are no signs of aggression, the ray is now completely satisfied with tank parameters and is moving around and eating healthy!