I had an issue when I mixed two 1.5 week olds with six 3 week olds. The smallest of the two younger ones (Barred Rock) got attacked a couple of times by my largest bird and it totally terrified her. I made a double brooder( basically a larger brooder with a piece of chicken wire down the center.) Both sides had separate water and food. After 2 weeks, I tried to integrate them, but the smallest one still got chased and pecked. So much so that everytime I checked on them, she was hiding in a dark corner and wouldn't go near the light, or food. I ended up giving her away to a woman in my town that I happen to know had bought some barred rock chicks at the same time and at the same store as I did. I've checked in on her and she seems much happier in her new home.
I hope you'll have better luck than I did.