Agree with Aart. Further comment: IMO, the sooner you integrate them, the better. Give the littles plenty of space where they can duck out of sight, but not be cornered. Fortunately, often when a chick is being pursued, the pursuit is forgotten the minute chick gets out of sight. Give them lots of entertainment while working on integration. (clumps of sod, scatter goodies in their bedding, multi level perching opportunities) Start by putting one or two of the bigs in with the littles... so the bigs are outnumbered and in unfamiliar space. Keep rotating bigs into play time with littles until they've all had a turn. This way you can observe to see if there are any bigs with bullying tendencies. Then, let them share space together. If you can set up a little play pen/run outside and start integration there, that will be best. When they're busy foraging and exploring, they're too busy to bully.
I am going to try to integrate my new batch of chicks earlier. They are now 1 week old and they will be going outside sometime this week.
I think the most important thing to keep in mind is that there should be no places where a chick can be cornered. I had this happen with my 5 week olds and luckily I was right there to rescue the one under attack. I have a 4 foot fence that has little doors at the bottom. The 13 week olds fly up onto the roof of the grow out coop, but they have integrated well with the younger chicks. Interesting because they know their status with the big hens. Looks like the little guys will know their status with the teenagers! Hopefully it will continue down the line, but I will take my time introducing the new babies.
I'm beginning to think I need to stop hatching eggs!