I would introduce them now, but be the momma to the young ones and keep the older hens away. Natural curiosity will get them together to check each other out. Enventually you can become less and less of a protector as a pecking order is sorted out.
My 5 week olds are out and around with the rest of my hens all day with out much supervision from me, and without any problems, but they have been getting use to each other for about 4 weeks now.
Yes, I took the one week olds out in the yard and let them run around until they got cool and started to huddle up. Then I took them in for the day. Mean while, the hens came up to check them out and I would push them away like a momma hen would.
I also think a lot has to do with the breed of hens you have. Mine are all gentle, so fewer problems.