15 adults including a rooster and 6 pullets and 2 cockerels at 19 weeks. Are those pullets laying yet? That can change the dynamics. I'll assume at least some of your older hens are still laying, you don't want to do anything that will cause them to stop laying in the nests. When the pullets start laying you'd probably prefer them to lay in the real nests. They can free range together but keep their distance. That's normal but there could be two different things causing that. You are trying to get rid of the two cockerels, I strongly approve.
How big your main coop is and how it is laid out is important, the larger the better. Since they have been housed next to the bigs and can free range together without incident I'd move the pullets into the main coop now so they are more likely to lay in the real nests. They may lay in those nests anyway, often pullets look to the older hens for guidance as to where to lay, but the odds of that increase a lot of they are sleeping in there.
The way I'd approach it would be to wait until dark and move the pullets to the main coop and lock them in. In the dark they are pretty easy to pick off the roosts. I just put them on the floor of the new coop in the dark and let then work out where to sleep. They always manage. Then I'd lock that old coop so they cannot go back to sleep there the next night. One of two things should happen. They may go to the main coop the next night or they may try to sleep near their old coop. You may have to move them into the main coop again the next night or for several nights until they get the message. Eventually they will. A lot of the time it only takes me once and it is done, but sometimes it takes a week or more.
The cockerels complicate it. I move my cockerels and pullets in at the same time but that is before the cockerels hit puberty, normally around 12 weeks of age. You can try moving those cockerels in with your pullets, it might work out. If you cant get rid of them I would but whether you move the cockerels or not, I'd be down there at the crack of dawn to open the pop door and see if there are any issues. What I normally see is the adults on the coop floor and the juveniles on the roosts avoiding the adults. It has always been peaceful for me but depending on your set-up and personalities of the individuals it can be risky. The ages of your cockerels is a bit worrying. Your rooster may go after them, I just don't know.
There could be two different reasons the two flocks are avoiding each other when they free range. Until chicks reach an age that they can force their way into the pecking order they usually avoid the adults. If they invade the personal space of a hen they are likely to get pecked. It doesn't take them long to learn to stay away. My pullets normally mature enough to merge when they start to lay. It's not an age or size thing, it's when they mature enough.
In the wild it is normal for the mature rooster to run the immature males out of his flock when they hit puberty and start harassing his hens. Normally they form a bachelor flock until one matures enough to create his own territory and start attracting females, but since yours were raised as a separate flock the pullets may just be hanging with the cockerels instead of merging with the main flock even if they have started to lay. There could be a combination of the two going on.
When I move my immature chickens into the main coop they are not going to sleep on the main roosts with the adults. If you think they stay apart during the day wait until they sort out where they sleep. Mine are most brutal toward each other on the roosts and that's with those that get along during the day. Expect your younger ones to find a safe but separate place to sleep until they mature. With a lot of people that can be the nests. Not always but it can be. It happened often enough with mine that I put in a juvenile roost, lower than the main roosts, separated horizontally a few feet, and higher than the nests. Hopefully your coop is big enough you can do something like that if you need to.
That's enough typing this morning, I've rambled enough. Let us know what you try and how it works out. Often these things go a lot smoother than we anticipate. Good luck!
