it depends on the individual personalities of the chicks. I've done it with no harm, but I've heard horror stories as well. Put them together, and watch them carefully. Have a back up plan in case it doesn't work.
I agree, it's not a good idea! Last year we mixed day old chicks with week old chicks in the brooder without a problem, I think we just got lucky. This year, the 2nd batch of chicks didn't arrive on time, so the chicks we bought last week on Tuesday grew pretty fast and are to big for the new chicks at a week and a half old. We had to build a divider down the center to separate them as the older chicks started pecking at the head, eyes, and feet of the new chicks. We'll give the new ones a week or so to gain some size and then remove the mesh divider, they should be pretty familiar with each other by then and hopefully we won't have any issues. So I say if the chicks are more then a week old, I wouldn't mix them!
I have found the the day olds are too delicate to mix with any that are more then 2 1/2 week old. A coupe of years I mixed some day old with 3 week olds and watched for while. The 3 week olds were stepping on the day olds. As the prior poster did, I quickly built a divided in my brooder and separated them. When the younger ones were a week old I removed the divider and they did fine. I will be getting chicks on Apr 6 and 13. Since they will only be 1 week difference in age they should be fine but I will watch them closely and will be ready to put the divider in right away if needed.
