I've been integrating 6 week old chicks in with my flock for a year and the older hens are fine, besides the normal pecking order. I always introduce 2 or more chicks at a time. I start by putting a large dog crate into the coop. The chicks sleep in there at night and range with the big hens in our super big enclosure. They eventually, after a few days or a week, put themselves to bbed with the big hens. I've never had a problem with picking or bullying, possibly because they all have plenty of room and food. Plus, I have several banties that are actually the same size as 6 week old chicks, so I don't think my big hens even notice the chicks all that much. My flock is so mixed and I add and take away chicks so often, that my hens are immune.