Last year I mixed my chicks in one small brooder and they varied in age by upto 3 weeks. I was planning to do the same thing this year. However I noticed that some of the biggest ones were stepping on the little ones quite a bit. This year I have a much larger brooder (3' x 8'). I put a wood divider and seperated the larger and smaller chicks for a couple of weeks. Each has their own heat lamp, food, and water.
The small ones are now two weeks old and much stronger. I removed the divider yesterday and let them integrate back together. There seem to be no problems so far. Most of them are staying in the 1/2 of the brooder they have been in the last two weeks. It is funny that the larger ones are afraid of the little ones. There is one little Deleware that ran over to the big ones as soon as I removed the divider. She started pecking at the big ones legs a little like she was trying to establish that she is the boss. The big ones now huddle in a corner when they see her coming. I don't see her being dominate with the other little chicks, just the big ones. One of the bigger chicks is by far the biggest. This one seems to be at the top of the pecking order for the bigger chicks. However, the little Deleware is controlling her to. In fact this was her first target when I removed the divider.
Side story - This big chick is interesting. I bought her and another at a feed store. They were supposed to be Blue Marans. The bigger one is obviously not a Blue. Looks like a Cuckoo Maran. I checked with the store I got them from earlier this week. They checked with the breeder and he said their father was a Blue Maran but their mothers were mixed Marans. I let them know that I was very disappointed that they are not Blue Marans as they advertised. An interesting thing about this chick is it is growing very fast. and is nearly fully feathered. It now looks like it is a couple of weeks older than the other chicks it's same age. Very odd. The other "Blue Maran" is feathering slowly (please don't be a rooster...). It is too soon to guess what it is going to look like. It is definitely going to have more blue or white than other.