I put my chicks and hen back with my flock at three weeks. So, it really does depend on how protective your momma is- when you put your hand in the brooder pen, how does she react? If she attacks you she will attack the other chickens when they get too close. Or does she step back and let you have your way with the chicks? If so, keep her and the chicks seperated for a bit longer. Your other chickens may care less about the chicks, you can always put them in and see how it goes.