We got ours last year about a week apart, and my bigger girls pecked and bothered the babies immensely. We had them in a big horse watering tank, and we ended up seperating them by a wire divider that we made out of PVC and wire, so that they could see each other and be living in the same brooder but the little ones could be safe. To this day, they are ok together in the coop but they are at the bottom of the pecking order.
We got 6 chicks on Saturday, 4 more on Monday and I will be getting a few more tomorrow and I think I am going to try taking my older 10 out when I put the new ones in the brooder, and then I will add 1 of the older chicks at a time to see if that makes any difference. Just a thought! Who knows if it will make a difference but its worth a shot.