I did the same thing yesterday, but I didn't do anything but leave them in the box for a little while, then I kept a close eye on them.
If I had to do it again, I'd have them in the same brooder, but separated by wire for a day or two. I had one older chick being a bully, but no chicks were hurt, and I had another older chick step up to protect the littles.
I am having the same issue! I have 3 16 week old chicks in the coop...3 12week old In the brooder...I range them together every day for two weeks...my older barred rock wants to kill them..oh , the 3,older I have a rooster...but the barred rock...my sweetheart...wants to kill the younger 3...two Easter Eggers and 1 Brahma....I thought I would put younger ones inn coop tonight and leave older in the run...all safe ...any feedback? Desperate!!