Ideally you would wait until the newbies are somewhat close in size to the older birds so that they can defend themselves. However, factors such as AMPLE space (free ranging is helpful), newbies outnumbering older birds, and even bird temperament (some breeds are just really easy going with newbies, while others are very aggressive) can influence, positively or negatively, this suggestion. It is a great idea to expose the birds to one another gradually though, through the safety of chicken wire at first is best...
How old are the new chicks? I add different aged chicks together by putting new chicks in a dog crate so they are in the coop with older ones but separate. After a week I open the gate and let them mingle and keep an eye on them but, haven't had any problems integrating.
Thanks for your advice. The new chicks I would like to add to my flock are just a few days old. My plan is to raise then apart until they get to about 8 weeks and then slowly integrate. My problem is that my hens are a mix of Rhode Island reds and white chickens from Long Island. I cannot tell what their temperprement is as of yet. I just built a roomy coop and can house a few more hens than previously thought. So, I think I will get three more chicks and slowly mix them in while keeping my fingers crossed.
Sounds like a plan so when the new chicks are older - If you can keep them apart by a tempary fence for a week in the coop (young ones will need to be kept in for a week to learn where to sleep) and older ones need time to get use to new addition to coop. After when you let them out together while free ranging the intigration will go much smoother.