I've had a Diadema (long spine urchin) in a 15 gal for 4 years because it was destructive in my reef. I feed it seaweed and even meaty foods and it seems fine. Once when i did not feed it for a week, it overtook a smaller urchin and chewed through its shell, eating it alive.
You could probably put another in the tank if you feed enough, but if they are like mine, anything slower than them will not be safe.