When to let Mom and babies mingle with flock...what has been said by others is right, let Mama decide. I was rather overprotective, and kept them separate, first in the basement, then in a dog crate that was closed. But as soon as I decided to open it, she took it from there, and probably could have much earlier. One reason I had waited so long was that it was Feb. 20 we got the chicks and they could not go out. Couldn't see them being in the coop with nothing better to do than torture the chicks.
But as soon as the door was open, Mama hustled those girls out to start training them. And she made sure that all the others stayed away.
Food. Give them chick starter. The rest of the flock will want it too as it is higher in protein than the layer feed. Around here in Feb. All I could find was medicated, which I didn't want (IMO medicine is a cure, not a way of life), so I wound up with game bird crumbles. I thought Mama should have some layer food, but she clearly did not want it, and repeatedly warned the babies away from it, so I removed it. Mama clearly wanted to teach them to peck things from the ground, so she scattered the crumbles all over, rather than teaching them to eat from the chick feeder.
Once Mama decided her work was done, the other hens were kind of menacing to the 6week olds, so they are in a small separate coop with a temporary fence within the larger pen. They all see each other, and these girls are supposed to be BOs, so can't wait to see everyone else give them space when they are full grown and can hold their own.
When you make changes, just be sure to be there to watch incase there is a real need to intervene for someone's safety.