I would tend to agree - wait until they are close to the same size as your current hens. I managed to integrate earlier - when they were about 7 weeks old and mostly feathered out, and out of necessity b/c I don't have a second coop or run, but it wasn't easy and it will depend on what your older hens are like. I sectioned off a part of the run so they could see but not touch for a week with a pet carrier as their coop. Then let them mingle supervised for a few weeks, but then back to their separate area when I wasn't around - (about two weeks). Then put them in the main coop at night and back to their own run during the day for about a week. Overall it took about a month. You may still have to do some of this for a while - depending on how the older hens act toward the younger ones. If it doesn't go well, the older ones can kill the younger ones. Not to scare you, just so you know to keep an eye on them.
For older hens that I purchased - I have just put them in at night and the others wake up and kinda say "hummm - I don't think I remember you, but you must belong here b/c you're here now and when I went to bed all was good with the chicken world"
(remember - quarantine for a month if possible - to reduce risk of introducing illness to your flock !)