I am not suggesting one way or another, but just telling you my experience.
I just integrated my 6 week fully feathered chicks into the big girl coop as follows:
first, I have 2 coops, one has three very docile battery hens. the other has the bulk of my chickens who bothered my battery hens, which is why i separated, the babies are being integrated into the battery hens coop, not the other one, and probably would not have followed the same plan if I had had to put them in with the more aggressive birds.
First, I put three babies in a large bird cage and set it out in the chicken yard for half the day. Then, I opened the door and let them come out when they were ready. Its a small door, so they could also get back in if they were scared. kept them out for the rest of the afternoon, then locked them back in their bird cage and put them in the unheated coop for the night. 2nd day, let them out with the big girls all day, and left their cage out in the yard so they could come and go, also made several diversions around the yard for a quick escape if necessary, and it has been necessary on a few occasions, and they did fine. Added 2 more babies, followed the same process for a couple of days, then last night they all spend the night in the coop, with a separated, yet accessible corner, and this morning they are just fine. Actually this is the second night for that arrangement. I have three more that are not fully feathered yet, that are still in the brooder. The reason I opted to put them out so early is because I thought the cold fall air would be better to acclimate them to the winter than in a few more weeks when it is really cold. Seems to be working for me, and chicks are happy and healthy!
Probably down to 40 at night here.