I personally don't believe the younger the better for everyone. We all have our unique set-ups, flocks, management techniques, and everything else that what works for one may not work for another. From what you describe though I would start very young. I think you can handle it but you may have challenges.
If your coop is too small to section off, how well will it work to sleep 8 chickens, especially with some juveniles and others fully mature? Some of that depends on how it is laid out, it's not just size, but them sleeping together can be a challenge. With mine until the pullets start laying they are not mature enough to share the main roosts with the adults, they get beat up when they try. So they look for another safer place to sleep. A lot of the time that is the coop floor though it can be nests. I solved that issue by adding a special juvenile roost, separated a few feet horizontally from the main roost, about a foot lower than the main roosts, and higher than the nests. Occasionally some will sleep on the main roosts early, there are always exceptions. How that works with you will depend a lot on how the roosts are laid out and the personality of your mature hens. As long as mine are not sleeping in the nests and are safe from predators I don't care where they sleep.
Can you get that tractor inside your run? At that size I doubt it but it might simplify things. Another possibility might be to park that playpen next to the main run and build a connection with a pop door between the two. That's how my grow-out pen is set up. If you do have to build something in the run, don't think coop, think shelter. All they need is a place to get out of the weather when they sleep. And fence off a portion of that big run so they can see each other.
My brooder is in the main coop. My brooder-raised chicks go there straight from the incubator or post office so they grow up with the flock. If I have enough room in the main coop I open the brooder door at five weeks. After they are all out I close the brooder door. They are now on their own with the flock. I've never lost a chick doing this but I have a lot of room in the coop and outside. Other than when they lay eggs or sleep the older chickens are not in the coop. When the chicks go outside they have plenty of room to avoid the adults. But this is not your situation.
When my main coop is getting crowded I move the 5 week old chicks to my grow out coop and pen. It usually takes me two or three weeks before they are all putting themselves to bed in that grow-out coop instead of sleeping in their run. Once I'm comfortable they will put themselves to bed in the grow-out coop instead of trying to sleep somewhere else, I open the gate to the main run and let them mingle with the adults. I have a 12' x 32' main run and about a 45' x 70' area inside electric netting so they have a lot of room. Usually around 12 weeks of age I move them or maybe just the pullets into the main coop at night, but my main coop is set up for that. After a month of rubbing shoulders with the adults outside I don't have issues. I've never lost a chicken doing it this way either.
Your set-up and conditions are different from mine. You may need to tweak some of this or do it totally differently. Hopefully you can get something out of all this that helps you plan it. A big part of it is to remain flexible. Good luck!
If your coop is too small to section off, how well will it work to sleep 8 chickens, especially with some juveniles and others fully mature? Some of that depends on how it is laid out, it's not just size, but them sleeping together can be a challenge. With mine until the pullets start laying they are not mature enough to share the main roosts with the adults, they get beat up when they try. So they look for another safer place to sleep. A lot of the time that is the coop floor though it can be nests. I solved that issue by adding a special juvenile roost, separated a few feet horizontally from the main roost, about a foot lower than the main roosts, and higher than the nests. Occasionally some will sleep on the main roosts early, there are always exceptions. How that works with you will depend a lot on how the roosts are laid out and the personality of your mature hens. As long as mine are not sleeping in the nests and are safe from predators I don't care where they sleep.
Can you get that tractor inside your run? At that size I doubt it but it might simplify things. Another possibility might be to park that playpen next to the main run and build a connection with a pop door between the two. That's how my grow-out pen is set up. If you do have to build something in the run, don't think coop, think shelter. All they need is a place to get out of the weather when they sleep. And fence off a portion of that big run so they can see each other.
My brooder is in the main coop. My brooder-raised chicks go there straight from the incubator or post office so they grow up with the flock. If I have enough room in the main coop I open the brooder door at five weeks. After they are all out I close the brooder door. They are now on their own with the flock. I've never lost a chick doing this but I have a lot of room in the coop and outside. Other than when they lay eggs or sleep the older chickens are not in the coop. When the chicks go outside they have plenty of room to avoid the adults. But this is not your situation.
When my main coop is getting crowded I move the 5 week old chicks to my grow out coop and pen. It usually takes me two or three weeks before they are all putting themselves to bed in that grow-out coop instead of sleeping in their run. Once I'm comfortable they will put themselves to bed in the grow-out coop instead of trying to sleep somewhere else, I open the gate to the main run and let them mingle with the adults. I have a 12' x 32' main run and about a 45' x 70' area inside electric netting so they have a lot of room. Usually around 12 weeks of age I move them or maybe just the pullets into the main coop at night, but my main coop is set up for that. After a month of rubbing shoulders with the adults outside I don't have issues. I've never lost a chicken doing it this way either.
Your set-up and conditions are different from mine. You may need to tweak some of this or do it totally differently. Hopefully you can get something out of all this that helps you plan it. A big part of it is to remain flexible. Good luck!
