What I learned is that since chicken flocks ALWAYS have a pecking order, any new introduction needs to be done carefully because the newcomers "disrupt" the existing pecking order and they will fight it out until the new order is in place. So... make sure the sizes are similar (don't put half-growns in with full-growns) and don't constantly add random new chickens without realizing there's going to be fighting, possibily disruption of egg-laying, etc, until the new pecking order is established.
Mine chickens free-range and are locked into the coop only at night, so if I buy new chickens, I have to lock them ALL up together for at least a few days to a week until everyone knows where they stand, and only then do i let them free-range again. They peck and chase and fight with each other during that time. If I didn't lock them up for that amount of time the "oldies" would chase the "newbies" out altogether and not let them come in the henhouse at all and they would not lay eggs in the nestboxes where I want them to.
