He doesn't hide at all haha, keep in mind hes almost 16 inches long though. I have provided him with lots of driftwood to hide amongst but he prefers center stage. Sometimes he sits at the back end of the tank and keeps a low profile.
For amount of sand? I'd be happy with 2 inches max, my next move before the ray comes is to take some sand out, as the larger fish have piled it all over the tank, reaching depths of 6 inches in some spots.
So the plan will be to remove some sand and either remove or rearrange driftwood so the ray has maximum floor cruising ability and the wolf still has a castle to retreat to if needed.
How much does your wolf eat and what is his size? Mine eats half a tilapia fillet every second day and shows no aggression to his tank mates which includes a handful of fish that are half his size.
Also, I paid $250 for the wolf and $200 for the ray, and I'm only slightly hesitant about this co-hab, for $25 I'd try it in a heartbeat!