have always been very cautious when introducing new/young chickens to the established group but I needed a break in cleaning/feeding so decided to mix the 2 month olds in with established 7 and 14 month olds. have 10 established chickens(no roos yet) and new bunch is 8 pullets/1 cockerel. they've done well together but only because the older girls go into their "trance" at dark and the youngsters, I have noticed, don't trance. can't remember when it starts but its helped the young ones to eat because the older ones are trancing. the youngsters get up earlier also and are in outside pen before the older chickens. have to gut the 12' x 8' chicken shed because of the new 9 babies , will need more nests/roost space. now that they're aclimated I can just kick them all out into the yard to play while I reconstruct the inside of shed. FUN!