I would wait at least 3-4 months at least and keep an eye on their behaviors before even thinking about changing there pasture buddies. A few years ago we had a stallion gelded and we had to wait for about two months, but was settled by the end of the month. He was a good boy, but wasn't stallion quality, due to some traites being shown after he covered his first mare. I'd say keep them away at least 4 months actually. Ask a vet. All horses are different.