You might want to keep them separate until they are ready to eat the same food as the older hens. Or switch everybody to something like Flock Raiser and supplement with oyster shell. At this age you definitely don't want the youngsters eating layer feed.
I would be be penning them up alongside the other's for a while anyway so they can all start getting used to each other. If you just pop them in there they are going to get the snot beat out of them. Make sure they all have plenty of room when you do put them together, or let them free range together for a bit each day before you try putting everybody in the same pen. Just watch them carefully, integrating new birds can go ok or it can be a complete nightmare.
