Mixing chickens of different ages is not recommended. Most of what I have read about this is because of the various diseases that can be passed to a new flock, or I assume could go from your flock to the new birds, but in all honesty, I would hope that someone would not introduce birds to their flock if they knew of a health problem.
I don’t know for sure if cannibalism is considered a disease but it is the single real problem that I have created by introducing birds into the flock. There are some suggested remedies like putting salt in the water or feeding scratch feed which has more grains heavy in fiber to eliminate this problem.
I always try to prevent problems. It is so much simpler than trying to cure them.
The interaction is very tough at first especially if the birds are much different in age. I mean this in the growing period up to possibly a year. At least after that if all the birds are mature, there should be few if any problems.
The birds should be put in the coop at night. I only tried doing it during the daylight once with a couple birds and at that time I had time to watch them, and they never did really mix. Even now I have an old Plymouth Barred Rock that is just a pet that I keep and she stands well off from the rest of the flock.
As recently as last year I introduced 4 hens and a rooster to the flock at about 3 months and they kept to themselves, although the rooster did go after other hens in the flock. When running free in the yard, the newer birds stayed to themselves and only mixed with the flock when going to the coop or running up to get food such as scratch feed that I tossed out on the ground for them. The hens didn’t fight and it created no problems other than it seemed obvious that they didn’t really fit.
The real problem will come from any roosters you might have. I haven’t experienced having a rooster that went after the hens until they were mature so my experience is that they won’t bother.
I don’t advise being convinced that it is a good idea to mix birds, but it can be done.
I do feel it is very important to mix them at night. That way it seems that when the birds wake up, they seem to almost think that the other birds had always been there. If they are put together in the daytime, I don’t really think they will ever mix and if they do, it won’t be very well.
Naturally if you are keeping them in a small pen you won’t notice much except that some might keep away from others but it would be very hard to notice. Outside they have room to just go far away from each other.
I did introduce leg mites to a flock by adding a couple birds. This was when I first began with my hobby and it was just a very stupid mistake. I had the birds given to me and didn’t recognize the scaly leg mites or actually notice them or I would have not accepted them as they were a gift from a friend. I also had zero ideas of what to watch for with any disease.