From what I've seen size is not very important. I've never seen an immature pullet stand up to a mature hen regardless of either's size. It's not the size of the chicken in the fight, it's the size of the fight in the chicken. In those circumstances there is no fight in my immature pullets, they just run away if there is any conflict. It sounds like you have a cockerel or two. There is no telling how those will interact.
That's not much of a quarantine. Diseases and parasites can spread by them eating or drinking from the same dish, from them pecking in the area where another has pooped, spread by insects such as mosquitoes, grasshoppers, or grubs, or even spread by the wind. You can spread diseases or parasites on your shoes if you walk between areas. The further you can keep them separated the better your quarantine. Most of us don't have the room or outbuildings to do that effectively. Some is better than none, at least it protects against a few things. The typical recommended quarantine period is 4 weeks. Some diseases take that long to show effects.
I like this. When you first let them together, this gives them the most space so they can get away from each other or just avoid each other. I do something like this with my chicks, I let them roam together during the day and sleep separately at night until I'm sure they are not going to try to kill each other while ranging. With my set-up a month is convenient but that's probably overkill.
As Aart said, sometimes that works and sometime it doesn't. Often these integrations go a lot easier than we make them sound, especially if you have a lot of room. A lot of old-timers just dump new chicks or chickens off with the free ranging flock and it works out. They may be OK with them sleeping in trees but at least they are not killing each other.
When I move my chicks into the main coop with the adults I move them in at night after it is dark so they wake up together. But I don't do that until they have roamed together outside for a few weeks so they re not strangers. When I do that I'm down there when they wake up at daybreak to open the pop door and let them out until I'm sure it's not a problem. With mine that's usually one or two early trips down there, it's practically never a problem. But I still do it, just in case.