Just imagine a chick at 6 weeks underneath a hen. And then more than one. Even a hen would say... enough. Mine were in the coop with just the heating plate at less than 2 weeks. And we are talking minus temps. They were just fine.They outgrew the heating plate we had, which is why we use the lamp instead! Good to know it's probably okay for them to be without it though. They don't seem to huddle under anymore during the day, not sure about the nights! Then again, they're in a shed, in a coop, so it's not like they'll be sitting in the cold wind even if the weather gets bad.
They're in a makeshift pen within the adults' coop, as they outgrew the coop we had them in previously (and it partially flooded twice...) It's off the ground, a walking plank leads up to their area. So the chicks can see the adults, but they can't interact through the "fence". None of the adults except for one really ever used the area, so it would be a safe spot for them.
Our adult chickens are 4 wyandotte bantams, a BarnevelderxSilkie cross and a large Brahma rooster. The chicks are already close to the size of the bantams! So they will not be smaller than the majority, in about 1 or 2 weeks. I'm really mostly worried about our rooster, Duts, since he is a big boy and I don't know how he'll respond to the chicks. He's very protective of his hens.
One option I have would be to put them in an entirely separate area, on the other side of our garden. They'd have plenty of space to roam, but they would then not be able to see the others unless we let the adults roam outside near that area. We were planning to keep the roosters in it once it becomes necessary to separate them from the rest of the flock, as it's a little further away from the coop.
And chickens don't care about size who is boss or not. Just saw that at my neighbors chickens. Adult bantam cochin rooster chasing teenage orpingtons at least 1.5 times his size. And they ran!!! That little stinker was chasing them left and right. They don't seem to see the size at all. My young hens last year were just about to lay or had started to lay. I don't remember. I moved 2 hens in with them that were a wee bit younger but not at the laying point. They were chased really bad until they started laying. They had the same size and it didn't matter.
You can try if they get along... and then make a decision. Maybe it will work for you. You can always separate them if you feel like it doesn not work out.