It depends on the weather where you live and how well-feathered your chicks are. Your chicks need to have enough real feathers, not the downy fluff, to keep them warm and dry. Different breeds mature at different rates, so you really have to go by feathering out. Additionally, if you just throw them in with your layers, the layers will attack them. They will need their own space out with the layers where the birds can see each other, but can't get at each other. This will allow the hens to get used to seeing them and hopefully not kill them when you have them greet each other later.