I combined a flock of 17 one-year old hens (and two piglets) with a flock of 100 6week old birds and they all seem to be doing great. I planned it out so that the younger birds were established in sight but out of reach from the older birds for about 2 weeks before mixing them up. They were in their own paddocks next to each other, separated by two rows of electric poultry netting. I would
feed them near the fences so they could get used to seeing and hearing one another at chow time. In this case the chicks were getting a little starter feed scattered on the ground so that they could learn to forage better. And since you can't give candy to one flock and not the other, I tossed a little to the older flock as well. I think this made them friends.
Then one night I moved the electric netting so that the paddocks were combined and enlarged. Everyone woke up and came out of their respective coops/pens and went to scratch around. There were a few little battles but nothing to be concerned about. If something gets rowdy between the
chickens, the pigs usually go to investigate and break it up. I think the fact that there were too many young birds to fight, made the older hens just kinda give into the idea of sharing the space. They still stay in separate coops though. At least til the roosters go off to freezer camp.
In your case I think I would be concerned about two things. Make sure the rooster isn't too rough in establishing himself with the new hens. They can be pretty brutal. I butchered our rooster before the new flock went outside to avoid giving him little targets to beat up. That also messes up the pecking order for the older hens so that when they got integrated with the younger birds, they really didn't have a "boss".
The other thing I would think about is a quarantine for the new birds. This way you don't accidentally bring illness into your flock. This is more of a judgment call but I like to er on the side of caution.
Best wishes