They can grow up in the same brooder. I mixed chicks together over a period of a month, and they all did just fine. I kept an eye on the last ones I added, because they were quite a bit smaller than the oldest, but no problems.
If they're young, from the same location, and just a week or so apart I usually keep them separate for a couple days. If no noticable health problems they go together. If older, from different locations, and separated by more than a week in age I keep them separated for a week or more before putting together. Hope this helps.
I ordered chicks from McMurray (they are the only hatchery I order from) and received chicks on May 23rd and May 31st and I have them in the same brooder and they get along pretty good because they were close to the same size..
I have some chicks that hatched this past Saturday and they are pretty small so I am keeping them seperate from the above mentioned chicks because of the size difference. I am going to try to introduce them in another week or so maybe. It will depend on their size then
I am new and I purchased chicks a week apart from the feed store and I did not have any other place to put them but together and they did just fine. Now they are about 12 weeks old and they are all getting along great. I just purchased 3 new chicks that are only 6 weeks not sure how they are going to do putting them in together,(they are in quarantine now) now just waiting for them to get big enough to see/