Hmmm, as a hatching addict, what I usually end up doing is setting up multiple brooders. I hatch new chicks every week. I stockpile 2 or 3 weeks’ worth of chicks in one brooder, and then the next group in another. Also as a chick-buying addict, whenever I acquire new chicks, I just fit them into whichever brooder has chicks about the same size.
As the chicks get old enough to not need heat, that’s when I move them into my “under the roost” brooder, where they can meet the adult chickens through the wire. After about a week, I just open the door and let the big chickens barge in. Sometimes there is some squabbling, but it doesn’t take long for the littles to learn how to avoid the bigs.
I think if the littles are feathered enough to not need heat, they are big enough to integrate. The thing is, I don’t kick them out of their space and fill it with new chicks. I let them have several weeks where they still sleep inside their brooder, even with the door open. Looking at your timing and the fact that you don’t have multiple brooders, I think spacing your groups out by 6 weeks will allow you to get the first group out and integrated before you have to put the next group in.
Ok, you posted as I am typing, and now it’s 3 weeks between groups. You are going to need a second brooder…