When to let newbs out with the main flock?
Do you mean that you don't have a run at all?
Well out in the part that you free range in, get a couple of pallets, and lean them against the coop. Put one on top of some blocks so that it is about 6-8 inches off the ground. These are hide outs and escapes for the little chicks. You can also put feed and water under this so as to make sure they get enough. Make sure there are at least two exits to any hideout, so as not to get a bird trapped.
Then let the big girls outside, and wait about an hour, while you set this stuff up. Feed the big girls a rather long ways away from the coop, and then let the little girls out of the coop. With you standing in between with a long stick so as to redirect the older hens away from the coop. Let the little ones explore. The will be quite timid if you have had them constantly in the coop since they were born?
This will allow the younger ones a chance to see where they can hide, and if you can, chase them a bit, so that they do find the safe place.
Then let them be. The younger ones will take some pecks, but should be able to get away