That is complicated isn’t it? There are many different issues there.
What is your weather like? How warm is it, mainly overnight lows? I’ve had a broody hen wean her chicks at three weeks age, but that was in the heat of a warm summer. Most places won’t be warm enough right now for chicks to not need supplemental heat regardless of which side of the equator you are on. Part of the concern is that they are not acclimated if they have been kept in warm conditions.
Some broody hens will try to adopt about any chick, but most hens and chicks imprint on each other and don’t want any others around. It’s possible your hens will try to adopt those chicks, but not likely. Even if they want to, the chicks might resist. Besides, could those hens keep that many chicks warm at night? Not likely. Three things are possible if you try to give those chicks to the broodies. They might try to adopt them, they might just ignore them, or they might consider them as rivals to their chicks and attack them. What I think is most likely is that they would ignore them, but when those chicks came round theirs, they would drive them off.
Chickens are living animals so no one can guarantee you what will happen. My roosters have never threatened a baby chick, some even help Mama out occasionally. But I integrate mine when they are pretty young. My brooder is in the coop, so the rooster is more likely to think of them as his offspring, not a rival’s. At three weeks I’d think you would be OK on that aspect with the rooster, but I haven’t waited that long for him to know they are there. Your rooster is probably not the danger.
The other hens are more of a risk. Contrary to what you read on here, they don’t always go out of their way to attack young chicks. Mine generally ignore them. But if a young chick invades the personal space of another adult hen, that hen is likely to peck it to drive it away. Sometimes the chick runs away and that is the end of it, but sometimes the hen follows to do damage. The hens are a risk.
I understand you need to get them out of the house. I have two suggestions. If you can partition off an area in the coop where the chicks can be safe behind a fence from the other chickens and you can safely provide heat, move them out there. You don’t have to heat the whole area, just give them a warm place to go to when they need to warm up, much like a broody hen does. She does not heat the universe, just warms them up when they need it. In about three weeks you can try to integrate them with the others. A lot of broody hens wean their chicks at four weeks, leave them alone to make their way with the flock after that. If you have a lot of room it may work, or you may find you have to wait a little longer.
Another possible way to do it is to set up a bigger brooder in an outbuilding or maybe your garage. A lot of people make one using big appliance boxes, taping them together and covering the top if they need to. At least this gets them out of your house.
Good luck. It is a bit complicated but you will manage.
