I added 6 leftovers to a new hatch of chicks that were a week younger, all was fine. I waited until the last hatched was in the brooder for at least 24 hours before I added the giants lol. By 24 hours in the brooder they can run and avoid, they all know where food and water comes from etc.
I added my single chick solo to my indoor adults with special needs because I needed his brooder for chicks. He was 3.5 weeks and the others adults, I actually had to remove little solo because after 3 days he started trying to peck the albinos red eyes. It all depends on if you get a jerk or two, but I think overall chicks get along fine.
I have added chicks of mixed age into one brooder - I think the widest gap was 10 days difference? I had to make a dedicated space for the littlies until they found their feet, then they co-mingled just fine!