I would absolutely wait until the older mare foals, and the foal is going STRONG before putting them together. I base this on the experiences I had when doing the exact thing you are talking about.
I had about a 50/50 reaction to my stud that had grown up with the herd. Some accepted him right away with no problems. But some did NOT take him seriously and they fought and fought with him. It took them several months to settle. They would stand by the fence and call and call for my old retired stallion who was 1/4 mile away. They could not see him, but they could hear each other's faint calls. This is now my third season with this younger stud and I still have one mare that just really doesn't want to be bred to him. She finally gives in, but she is not happy about it. Other things to consider:
1) Although your stud is probably mature enough to do the job physically, he may not be ready to take on the new position in the herd socially. Sometimes they have a hard time finding the courage to be aggressive enough to earn the position. They are used to obeying these mares, not going against them, and they just might be too shy to force the issue if the mares put up any resistance. This is what my stud did at 2 yrs old. He did manage to get a couple of the calmer mares bred that year.
2) Conversly, if he is ready to assume the position, he will not have any manners at all. All he will have is a GO button! Around and around they will go. He will chase them unmercifully. You might see the mares dribbling urine down their legs in an attempt to show submission, or in an effort to get him to realize they are not quite ready. He will not know enough to wait for just the right scent to get his timing right. Sometimes even if they do stand for him, he tries innovative positioning and then gets mad at them when it won't work! Since the young mare won't know what she is doing either, it will be up to the older mare to teach him the appropriate behaviors. She probably won't tolerate his aggression and ineptness. There will be a lot of fighting and you don't want the foal caught in between.
3) I had the best luck hand breeding the young mares to the young stallions. It seemed to work best to let them both learn the program in a controlled situation. With the older mares foal to consider, it would also be safer for the baby to hand breed.
For what it is worth, I have been around several 2, 3, and 4 year old stallions many times and I think the three year olds were the worst behaved. They were old enough to be very, VERY ready to do their jobs, but young enough that they had not yet been taught any manners by the mares.
Every situation is different so you may not have any of these things occur, but at least you can be prepared for them if they do! Best wishes in your integration efforts!
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