Your younger ones are 8 weeks old?
I would do just about the same thing as the reverse. Make a place in your coop that keeps the birds securely separated. Let them live side by side for a couple weeks... feeding them close to each other so they can all see each other eating. Throw grain on the floor close to each other on both sides of your separation. Then after you've done that for a while you'll want to let them out together you can throw grain for them but scatter it widely. You can probably tell who is the lowest on the pecking order of your older girls. Some may suggest taking that bird and exposing her to the younger ones first but from my observations, I think the lowest has the most to lose and may actually be meaner than someone higher up.
Are the young ones still peeping? Sometimes that will make a difference in how the older girls treat them.
However you decide to do it, you're going to have to watch very closely and be right there with them when you allow them to be out together for the first several times. Roosting always presents a problem too, they're very picky about who gets to sleep next to them. Perches in steps are probably best. That way the birds which are higher on the pecking order can take the top perch while those beneath them can take their places beneath them on the lower perches.
