I've only ever had it happen with older chicks picking on a younger chick. I used to hatch at different times, so that the chicks could be as much as 3-4 weeks older than the newest ones. After the first time, I sectioned off my brooder. Made it like 3 stages. Stage one, higher temp, for up to a week old chicks. Stage two, lower temp, up to 4 weeks old. Stage 3, they stayed there until the weather outside permitted them to move to the chick tractor (to keep the free range adults from getting after them)
When I wanted to introduce chicks to the coop, I had various "floors" to my coop. The top floor was where all the higher ranking fowl slept and where the hens nested. Below that, subordinate birds. Below that, newbies. Each with their own door. New chicks went into the bottom one, and stayed there for a week. Then they were allowed to free range, and fight their way into a higher level when they were feeling confident. One rooster, found his way to the top in his first year without beating up anyone too bad. Another rooster decided he was going to the top fast, and I culled him, for being way too aggressive. He showed the potential to do some serious damage and he wasn't that nice to the girls.
You can do things to help transitioning birds, and to keep them seperated at various ages, and having enough space to start with is also a help. It's worse when they're crowded.
And never, ever just throw new birds into the flock to fend for themselves, regardless of age or gender. It's also good to brood full size chicks seperate from bantams, unless the Bantams are older then they deal pretty well.
