Once your group of geldings is stabilized, it is probably safe to turn your colt out. Given that he is still quite young and probably fairly submissive, the older horses are unlikely to hurt him.
You may want to first introduce the colt to the boss gelding alone and let them form an alliance before turning them out together with the other two geldings. That way the older horse will be more likely to protect the younger one.
Another possibility is to keep the colt in a pen adjacent to the pen the geldings are in so that they can become acquainted over the fence. There will still be a period of adjustment once the colt is turned out with the geldings, but it may be less of a ruckus.
It is a great idea to let stallions hang out with geldings whenever possible, but this generally has to be started at a young age and before the stallion begins his breeding career.
Also, horses don't really have the same concept of size that we do. It is very common to see a pony be a boss in a herd of bigger horses because so much of it is attitude. However, horses do have a concept of youth and are less likely to be aggressive to a youngster. A horse under two years old will display a behavior called "chattering" or "snapping" which signals to the older horses that "I am just a baby and don't know any better, so please be gentle."
Julie Goodnight
Trainer and Clinician
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