What sexes are your chickens? Are all of them females or do you have some males in the mix? The answer to this is not going to change my answer much but if there is more than one male involved your chances of seeing more serious fighting goes up.
You have done the basics, house them side by side so they get used to each other. I like that you have let first one group them the other range separately. My next step would be to open everything up and let them range together. Let them go back to separate sleeping places if that is what they want to do. That’s generally what mine do. On extremely rare occasions I’ve had chickens move into the main coop on their own but usually they go back to their own coop.
Only after they have been ranging together for a while (I usually go a month for this step but that’s probably excessive) I lock the smaller coop and move them into the main coop after dark. Mine try to sleep near their normal coop but are usually really easy to catch after dark. If you have lights out there like street lamps or security lights this catching them after dark can be a bit more challenging.
How big is your main coop? To me that is an important part on how easy this goes. The more crowded they are the more likely you are to have problems. In my opinion, room is a big part in why some of us get the results we do. What works for me might not work for you. Often integration goes so smoothly you wonder what all the worry was about, but occasionally you do read about a nightmare. I think the more room you can give them the better your chances for a smooth integration are.
What I expect to happen with yours when you let them range together they will probably maintain separate flocks. They will pretty much ignore each other and just stay separate. If you have multiple males there could easily be some fighting but usually they quickly decide which is the boss and go their separate ways. Some may fight to the death or one may get injured, but with the space you have they usually reach an accommodation. Free ranging makes this so much simpler.
When you merge the flocks with your age difference there is a pretty good chance the older group will not share the main roosts with the younger ones. Some mature birds can be pretty brutal toward less mature birds on the roosts. The younger ones often look for a safer place to sleep. That just might wind up being your nests. This happens so often with mine that I put up a juvenile roost, lower than the main roosts but higher than the nests and horizontally separated from the main roosts so they have a safe place to go that is not my nests. Normally my pullets start moving to the main roosts about the time they start to lay, though that can vary
When I first lock the juveniles in the coop at night, I get down there pretty early the next morning to open the pop door so the young ones can get away from the older ones if they need to. They hardly ever need to, usually after the second morning I’m satisfied it’s going OK.
When I go down to open the pop door, I normally find my adults on the coop floor and the younger ones on the roost or hiding under my nests, those are pretty low. The younger ones are avoiding the older. The bigger your coop is and the more places for them to hide from the older ones, the better. If you were restricting them to a run I’d suggest putting stuff in there they can hide behind or perches where they can get away from the older ones, but since you free range, that should not be necessary.
My chickens are a lot younger than yours when I go through this. My brooder is in the coop so the chicks are raised with the adults. Sometimes at 5 weeks I just open the brooder door and let them mingle, they do fine. More often I move them to my grow-out coop with attached run until they are about 8 weeks. By then they are going to bed in the grow-out coop and they are too big to get through my electric netting so I let them range with the main flock. Again, no problems. Sometimes I move them all in the main coop at 12 weeks or later. Sometimes I move the pullets in the main coop but leave the cockerels sleeping in the grow-out coop. I want all the pullets sleeping in the main coop before they start to lay, they are more likely to lay in the nests that way, the adult hens show them what to do. You might say I’m pretty inconsistent in what I do, but I look at it as being flexible to changing circumstances.
Good luck! The majority of the time this process goes rally smoothly especially when you have so much room outside.
