I have mixed chicks MANY times. I hatch non stop for 4-5 months straight. I have a few huge grow out pens. With each hatch I raise the chicks in the house for up to two weeks then add them to the grow out pen. When I have 4 groups ( 3-4 weeks apart) going and I go to add the 4th hatch group the very first group goes out to free range as they are 12 weeks old and old enough to be outside in the BIG BARN! I have never had an issue except once. I added the chicks to the pen and didn't have time to let out the older ones until the next day. The little ones stayed in the corner scared to death and wouldnt eat and drink until I removed the big ones!!