I have 2 flocks of chickens that I would to merge into one at some point in the future. 1 flock is 5 Road island Reds with an Unknown age and the other is 18 pullets that are 5 weeks old which breeds are Barred Rock, Buff Orphington and Black Astrolorp. I am not sure what other information I should provide at this time.
If you have an enclosed space that will hold both flocks comfortably it can be fairly easy to integrate the two flocks.
The enclosure will need a divider fence good enough to keep the flocks apart but able to see each other for a couple of weeks.
Using this method means they will be able to get used to each other without the ability to peck or other wise antagonize each other.
Once they seem to be ok with each other, remove the divider fence and let them sort their pecking order out and everything should be fine.
I wish I knew this for sure but I read in a chicken book recently that when the young birds are at a certain age (I want to say 6 weeks but don't quote me) it's the best time to integrate with older birds. At that particular age they are big enough to not get pecked to death and young enough to accept a lower spot on the pecking order without much fighting. I have no experience with this, I just remember reading it in a book from the library recently. So I'm not positive what that age is but I think you're close. Good luck!
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"
I recently integrated my flocks and have not had any problems yet. I did basically what Bryant suggests.
Once my chicks (17 Eastereggers) had their back feathers, I removed them from the brooder and built a wire pen (nursery) with one of the sides was a common side to the run/coop where the adult birds were (22 adult Orpingtons and Plymouth Rocks). Not only are these two different flocks, but different breeds that have different behavior characteristics. I kept the chicks in the nursery for three weeks where both flocks could see each other. I've read that we should wait until the chicks are of similar size as the adult birds so they can defend themselves. However, it wasn't size that was the determining factor for me, it was speed. My new Eastereggers are far quicker and faster than my adult birds. The big girls can't get close enough to the chicks to peck them to harm. So, I integrated them at night when the adult birds were sleeping and roosted. I carried each chick by hand into the run/coop and wetted their beak on the nipple waterers because they were drinking from a different kind of waterer in the brooder and nursery. I went out the next morning and the chicks kept themselves segregated from the adult birds and would run away whenever an adult bird headed their way. A week of this and the chicks started to get used to the adult birds and got a bit more brave each day. They now venture out of the run to free range on their own now like the adult birds do, but they don't go nearly as far. They still feel secure being within a few second scramble back to the run. Where the adult birds are 2 acres away. With each day, the flock is getting more and more socially integrated. The only change I would make to this process was to install a second feeder in the run which I will do this weekend. But, I want them to get most of their nutrition from the 3 acres they free range each day.