When should I intergrate new chicks with old hens?

It seems to work best when they are very close to the same size.

I actually integrated my new chicks with my adult hens a couple of weeks ago. My chicks are two months old and not as big as my hens BUT... they are only together during the day in the very large run or when they are all out free ranging. I have them in separate sleeping quarters at night still.

My hens will occasionally make a run at one of the chicks but there is so much space outside for them that the little ones can get away easily so it's worked fine. I wouldn't have done it had I not had the space that I have. My run is about 650 sq. feet.

I also have one little chick that was hatched under my broody. Momma decided she was done being a momma (during the day time) after a few weeks so that baby is "on its own". I sectioned off a small part of the run for that baby with a dog kennel panel. Only that chick is small enough to get through the large chain link and she goes in and out at will. If anybody (the older chicks or hens) tries to mess with her - she just runs back in there and is perfectly safe. She has her own food and water in there also.

Interesting, that baby still roosts with her momma at night, sleeping right under her and no one messes with her. So, momma is still momma at night only. Go figure.