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If they have plenty of room, they should be fine. Mine were. I added 9 day-olds to a group of 3 week-olds, I don't know if it was because they were outnumbered or what!! I also added to that group 12 day-olds, when the youngest of the first group were about a week old, then added 4 more 3-day-olds to the group, when the youngest was about 1-2 weeks, and everyone is doing just fine!!
I plan on adding my coming babies to the mix as well. I will wait till the new ones are a few days old, and getting around pretty good before tossing them in. I usually wait till they start to smell like chickens before I take them out to the big brooder!! Right now the birds in the box are a mix of standards and bantams, from 1 to 6 or 7 weeks old.