I found it almost impossible to introduce my 3 month olds to my 5 month olds.I put one in not expecting much of a fight and i watched closely they just about killed her..they pecked her comb really bad and thier was a peice of flesh on one of my 5 month olds beak
So i have plans to rotate my hens during stages in thier life,since i am going to soon be raising 40+ birds or more.This has worked well so far..
At 0-4wks(until feathering gets going good) raise them in a small brooder low roof,95deg,or so.moving heat lamp to reduce temp 5 deg,each week( in barn ,but chicks cannot see out)
At 5-12 wks in a larger brooder 3 ft tall or so so they can have a place to fly up to roost.Windows so they can see out(i put this inside the barn also, so they can watch the older hens)
At 13-18 weeks in a chicken tracter/coop(homemade rig,outside barn for one i needed more room inside barn,for two they can still see the older hens when the older hens go out to range)
Then 19-on up,Let them free range all day, and lock em up in the barn at night.
This way all my birds get to see each other but not hurt each other till thier old enough to stick up for themselves.
That is what i do,so far it has worked excellent.