Here is my group of yearlings and two year olds. BUT, keep in mind, they have been together since day one (before they were weaned so mom could establish "rules" of play). I have no problem. Mine are ponies, might that be different? PWynn, I know you have ponies.
I had a pasture for young colts and geldings and had no problems. Mine were ponies too. I had a four year old gelding, a three year old gelding, a two year old colt and a two year old gelding, a yearling colt and a weanling colt. They all did fine but none were at all aggressive. They had a pecking order and played around, of course, but no one challenged anyone or got hurt. It was a big field with plenty of room for all though.
And one last question, if anyone here has had a retained testicle like this, could you give me maybe a rough estimate of the cost to get it out, along with what might be found in there that would impact the cost?
Can't advise on turn-out options without knowing the horses. But surgery for this sort of gelding runs about $800-1500 in my neck of the woods.
We've always kept mixed age group herds. I think with your situation the problem is timing and the fact you have that '09 one already marked as a rough player. If they haven't had a chance to socialize before I'd be very cautious.
I would also not turn out newly grouped ones in a paddock but only in a large field where the underdog can put enough of a distance between him and the rest. It were different if you had more than just the one '10 colt to integrate. The bully would wear himself out pretty soon if there were 4 colts to chase instead of just one.
I my experience whoever gets newly introduced will have a few tough days mostly because they get frightened of the big boys unnecessarily as most of them will not turn aggressive. But if you have one you already know acts that way I wouldn't throw in a single '10 with him.
I would wait until that bully was gelded and put with some mare-bunch that would get him mannerized.
The boy-group we currently have is 7 heads with the oldest being a coming 4yo (teasing stallion on his winterbreak), a 3yo that was brought up entire and gelded a few weeks ago, and colts of younger age. No weanlings in there yet as I keep colts and fillies together through their first winter. That way I can easier monitor them and feed more individually. In the spring they boys will then join the boy-group. At the same token most of the 3yo's leave the group to begin work and only the older boy stays around. I've never had an adult stallion turn aggressive. The ones that turned out unsocial always did so by 2yo and I'll instantly kick them out for peace as soon as they do. There is no room for unsocial horses in a group setup.
Interestingly I only had to do this once. It was a colt that had come as a weanling and was sort of dull in every regard. With all the others it was always transitional and everyone else has blended in after a few weeks. I think the more stable the herd is in itself the lower is your risk to encounter a situation where one won't fit in and keep tormenting his unders.