If you do not have a large flock and are not over crowded, you can leave them together. I have successfully done it three times. Many do separate them, but you do not have to.
The first time, I moved the broody hen, and she moved back, abandoning the nest I wanted her in. She got out, and the eggs died. She went back to 'her' nest, and I left her, at approximately 21 days, I slipped 6 chicks under her at night, and she raised them right in the flock, protecting them from the others.
The second and third time, I just left them were they were, it was a small nest and none of the laying hens laid there. So I did not have trouble with extra eggs showing up in the clutch. It was July when they hatched and for 24 hours, I did lock out the laying hens over night so that they would not kill the new chicks, while she was still waiting for the other chicks to hatch.
After that, the broodies would usually stand between the chicks and the other hens. It was summer time, and they all did quite a bit of free ranging, and had plenty of space. As the chicks got older, they would get an occasional peck, but for the most part they learned to stay out of reach.
So it can be done.
MrsK