we had three for about a week and a half, went and got one more and just put her in there with the others (the farm store girl said no problem as long as the first ones were under 3 weeks old) - so we did, and it was fine. In fact, my bossy pushy chick stopped being a pain and calmed right down. I guess the new chick became top dog to her
We mixed age groups in the same brooder twice this spring. We went by the safe to mix ages up to 3 weeks old rule. We received three 2 day olds and one died a few days later. So a week after getting the first chicks, we added three more chicks (two 4-5 day olds and one 1 day old) because minus 1 equals plus 3 in chicken math!
Then two weeks later we added four more chicks (one 5-6 day old and three 2 day olds). Integration went smoothly each time. We made sure with the last group that the babies had somewhere that they could go to escape the hooligans. There was a very small amount of pecking on the first day but the little ones were scrappy and pecked right back! The older five were actually afraid of the little ones when we first put them in the brooder! It was funny to watch the bigger ones huddle in the corner while one of the little ones gave them attitude! The little ones used the hiding spot once during the first day and that was for a nap to recover from their trip. After that, they never used it so we took it out of the brooder. We are just hoping that everything goes as smoothly when it comes time to integrate this year's babies with our existing flock of hens.