He will be 'proud cut' being gelded that old
We simply have not found this to be the case. We have had few proud cut geldings at all and the worst ones we have had were gelded at 2 or younger. There were 3 or 4 that we have had that would actually 'cover' a mare in heat. Two were so bad that we had blood drawn and Testosterone levels checked. We were sure one just had to have a retained testicle as you could hand breed him. Blood levels came back normal. The Vet school we took the one to (Colorado State you.) said that some geldings are just that way and they did not have an explanation for it. That was the last time I have paid for blood tests on any gelding.
It will take 30 days for the residual testosterone to leave his system.
This is just not true. When you turn studs in together or turn them in with geldings, their Testosterone levels drop within 48 hours.
he needs to stay by himself for a little while after surgery.
This is why many horses do not socialize well after gelding. You need to put them together (with no mares) as soon as there is no danger of them bleeding. I had one break open and bleed -- a lot -- when I put them together right away. One day separated seems to work very well.
The old thinking that stallions needed to get the Testosterone out of their blood has been proved a myth. We always thought it was a myth because, like other big ranches, we would run stallions together and with geldings during the fall and winter. Then, a Vet School, (I believe it was Texas A & M) wondered how big ranches could turn all their studs in together when they came out of the pastures in late summer without them killing each other. They went to some of the biggest ranches in Texas and did blood studies. They pulled blood the day they were taken out of the mare herds. They were turned in together and blood taken every 24 hours. They found that Testosterone levels dropped almost to the same level as most geldings within 48 hours. When we have turned them all in together, we have found that within 2 days, our studs quit grunting at each other and acted like a pen of geldings. We have had studs become 'herd-bound' buddies.
Like I said before, the whole key to gelding mature stallions is to geld them and get them in with gelding as quickly as possible, preferably when they are still sore and feeling sorry for themselves. If it is practical, we like to turn them in with geldings before they are cut. Just don't put mares across a fence from them.