Do not put them with the older birds until they are almost the same size, the older birds will be really rough on them and can even seriously injure or kill them. I don't integrate until 12 to 15 weeks old. Until that age I keep my youngsters in a grow out pen with it's own coop right alongside the older girls. The little ones move from their brooder into their grow out pen at 5 weeks old. So the older ones have a lot of time to get used to seeing them around before there is ever any mingling. By the time they do mix pecking is very, very minimal.
You just really want to take it slow when integrating new birds. Chickens do NOT like newcomers, at all, and if you try to go too fast the more problems you are likely to have. I know some people advocate the "stick 'em the coop at night" approach for new birds. For what it's worth, this has never, ever worked for me. The older birds are smart enough to know there are newcomers when they get up in the morning and unless you are right there to supervise before they get off the roost it can result in injury to the new birds. Slow and easy and you'll end up with a peaceful, combined flock with a minimum of stress to all. Including yourself!
