I have paired a 6 month old colt with a older, gelding that is used to herd situations and is safe with youngsters. That worked extremely well. I did move the youngster as a 2 year old gelding in with a mare family when he got a little too big for himself. The alpha mare promptly showed him what his manners should be. It could have been a dangerous situation if I did not carefully plan it out, and it was fully supervised.
Putting a young colt, hopefully gelded, in with a mixed herd is not without risk.
The best solution that will suit you will factor in how much you are willing to risk.
If the colt is extremely valuable, then you might select a more conservative approach and just use a known safe horse as a companion.
If the colt is not all that valuable, then you might assume more risk and turn him out while hoping for the best. He should be gelded first, of course, and fully recovered for at least 30 days…maybe longer.
An interesting side note, I had a discussion with a German Verband and was told that all of the young colts are turned out in big fields to grow up. I asked about the risks and loss of potentially extremely valuable stallion prospects. I was told that they breed so many of them that they just assume the risk knowing there will be losses.
Just my thoughts…