maybe let them meet through meshing or fencing for a while so they know who each other are but can't hurt each other. I think bum nipping is normal and not too rough, I am new to bonding, but allow bum nips, i break them up when they start boxing or lunge for the neck area at the moment. If u are worried about the size differences etc maybe keep them side by side in pens until your baby bun is bigger? Also when baby bun gets older it may need neutering as could make the relationship turn sour with territorial issues.
Are they house buns? Did u introduce in neutral area? This the the rule that most people go by and will advise on here.
Hope this is of help
Good luck