Knowing when to let them out is a tricky question. Alot of it is down to the Mother hen. Hopefully she isn't the lowest on the pecking order when she's with the other hens as this can make it more tricky. Is she defensive of the chicks, will she protect them from the other hens?
If she will, you can let them out from around 4 weeks, that way, they are fast enough to run away but can take a peck or two. Some feather pecking is expected, but any blood, remove them all and start integrating again slowly.
I had my broody in a smaller pen within my main run so the 'see but no touch' method of integration allowed the adult birds to get used to the smaller ones. This was in place from hatch until around 4 weeks when I started letting them all out together.
When I let them out, Mother hen defended the chicks and showed them where to go, what to eat, scratch etc and was still interested in looking after the chicks. I let them out in intervals, firstly a couple of minutes, then half hour, then an hour all supervised. Then I let them out all together for longer and longer periods without supervision. It went slow, but minimal fights happened.