I added my 8 week old in the general run, but with a sturdy divider that allows them to move back and forth, but the large hens are stuck on one side. Free range, I let them all together. Hens will kill chicks, so you want to be careful. Also I have trouble introducing bantams to an established standard flock.
I also have this question(when to introduce my chick to my other birds). Of course I only have 2 hens so I am hoping it will be easier then with a large group. I like the idea of having an area for them to run to and escape the hens if they get to mean. Mine were born middle of April so can I go ahead and introduce them to my other 2 hens now? I have 5 chicks so they do sort of outnumber the hens, lol.
I forgot to mention that it helps if you let the two groups hang out within sight of each other for a week or two, but separated before introducing the groups. I had the hardest time introducing a mixture of 3 standards and a bantam to my two RIRs, the first time around. The second group that got introduced was four Marans The bantam accepted them in a day, but the RIR took monthes, though they still like them better than the bantam.
4roosandahen - I think some hens are quicker to accept than others.
I try to mix my chickens when they are all about the same size. Chickens do have a pecking order and I have found that older established hens will pick on the littler pullets. I would try to separate them with a fence first and let them work things out with a barrier first (to protect each party). than after a few weeks you can try and put them together. Just watch them those hens can be especially mean to chickens smaller than themselves.