When I had the chickens in a smaller area, I always waited till they were about the same size also. However, I now have them in a huge fenced in area where there's lots of room to roam and I have put in some small ones (as young as a month) and they are fine. They are at the bottom of the pecking list of course but not overly picked on. The hardest part about mixing them is not being able to feed them separately.