Yes, i have integrated before, i set up a barrier around the smaller coop to keep them separate for a few weeks, then opened it up to provide a place that was too small for the adults to get into but the babies still could access, then took away the fence. Took several weeks to do it this way. At the time I also had a young but good rooster who acted as peacekeeper and protected the pullets coming into the flock. He has since died.
Both coops are being used but since the raccoon attacks, the flock has been reluctant to go into the larger coop except to lay. When roosting at night, they will often end up on top of the run or large coop or cram themselves into the smaller coop. i am afraid if all they have is the large coop to go into at night, they may not want to go in. It’s not the room i am worried about, it’s making sure they are old enough to survive outside in sub freezing weather.