Cone on rat after tumor removal surgery?

My rat went and got a golf ball sized tumor removed from his hip (its suspected to be bone cancer) and the vet put a cone on him. I looked around and saw that most people said that cones don't really do anything, but I wanted to ask anyway. He got stitches under his skin and his wound was glued shut and has been given pain medication for the next week or so. Would I need to worry about him harming himself if I took the cone off? Also, should I put my docile elderly rat in with him? Here's a picture of his surgery wound if it helps any.