We've got a flock of only 8, though it went to 12 at one stage. It's been a mix of rescue, bought, donated (long-term fostered) and found (cockerel on the lane!). In my experience the best way was to make a temporary home up for the new chooks right next to the existing ones. For example, we had a couple of new hybrids so I built a nest box out of wood, with a perch on top, and that all fitted into a large black compost bin. This was then placed into a run about 4 square meters for the two chooks for a couple of weeks, with some of that green workers fence mesh (usually you see it orange-coloured by road works), in between them all. This way they got to see each other for a few weeks. Then we let them range together, although they're in a large grassy paddock, but they used to go back to their own beds. Eventually we removed their compost bin and plopped them into the coop at night time, making sure we were up early enough to open it before fights ensued inside, for a few days until they got used to their new home, and the existing chooks to them.
Whilst they're all one flock now, under the jurisdiction of the cockerel who just ended up living with us, at one stage we had our original 4-bird coop, an 8-bird coop for some foster chooks we took on (and the lady never came back), and the compost bin for the new arrivals. All would range together in the day happily, and go back to their own coops at night. Lots of fresh grass and/or different feeders were the answer I think - space to get away if needed and to avoid confrontation. Now they're all in one large 12-bird coop and tend to hug each other down one end like a big happy family
Mind you - the cockerel does keep them in their place - best thing that ever happened to us all - I'm amazed at how caring they are to their ladies! Last to eat, calls them over if he's found a juicy worm - he picks it up and drops it and squawks, pointing to it for them. And then when they're eating he'll stand guard. A right gentleman (well - unless he's howsyerfathering, in which case he's a bit rough)