The two Dominique chickens a roo and a hen are about 2 years. The two Golden Sebrights are the same age. I have 3 Guinea keets that are 14 weeks old and getting a run all to them selves shortly. I have 13 pullets of 10 weeks in the run with the adults they don't seem to care until roosting time and need their space. And they give them their space after a couple of dissapproving pecks from the older hens. They all free range throughout the day.
You should probably introduce them at about 3 1/2 - 4 months of age, about when they get full-grown. Introducing a small amount of 2 month old chickens to a larger amount of adults in my experience can lead to some deaths. Doing it the other way around is just fine.
There is a huge difference between a 6 week old chick and an 8 week old chick.
Also there is a huge difference between an 8 week old chick and a 10 week old chick.
By this I mean in their response to grown up birds. They peep like babies, put their tushies in the air, and hide their faces in a corner when they are too young to be in with aggressive older hens.
What matters is, will the older hens give them a peck or two and walk away? IF this is the case, then it is just the normal pecking order business. If they look like they are about to kill the babies, they just might.
I would definitely supervise all attempts carefully to integrate the flocks and it might take several attempts before they are old enough. I speak of this because some folks are desperate to integrate due to the fact that they don't have the facilities for a separate coop and run, and are really tired of chicks in the bathtub.
I would say that it is infinitely better to integrate the latest you can manage it. And personally I would do it at 3 or 4 months (4 is better) due to the fact that you only have two joining the flock. Earlier might work, but it will depend on the personalities of the older hens. I have sold hens for aggression and I know that if they were presented with babies of 6 weeks old they would have killed them. I pulled mine out as I did indeed try it.
I mention 6 weeks, 8 weeks, 10 weeks, and so on, since I presume that is when you are going to consider integrating for the first time.
I have a different set up. My babies stay with their mothers, with the rest of the entire flock, their entire lives. I have plenty of space, and I've NEVER seen a chicken KILL another chicken. Some babies do get stepped on, but I've come to take those losses as the price of doing business. Like I said, I have a huge coop, and they free range. I say, if the babies are old enough to run away, and have plenty of room to do so, you'll be fine. I've raised hundreds of chicks this way. It works for me. I think plenty of room is key. Most of my hens don't even care, except of course when the day is over and it's roosting time. All of the young ones just huddle together in the corner on the floor, away from the roosts. I have gentle breeds also, black giants and australorps with one golden comet and white leghorn thrown in. Maybe 60 birds or so right now with more due to hatch in a couple of weeks.
Thanks for all the info! I think I have decided to wait to introduce them until they are older. I have them in a different run with their mama. Or, should I say their adopted mom. She is a bantam. They are dominique. And likely to get into trouble with the larger hens protecting her babies anyway. I appreciate all the advice!