hi, i have 19 chooks and 2 ducks, been keeping hens for about a year now, and slowly get more as i cant say no to a lonely poorly chook.
My advice, and this is purely what i do personally...
i have a large enclosure for my chooks, but i also have a smaller enclosure built at the side of it with a seperate house, so when i get a new lady or ladies they go in the smaller enclosure, seperate from the other chooks for 2 weeks, to watch for any developing problems or signs of illness before they go in with my other chooks.
They can see each other through the wire fencing but cant get to each other, this way they get used to seeing each other, then i introduce them daily short periods at a time, they will argue, they need to establish the pecking order between themselves, only intervene if it gets out of hand, ie blood.
They will live happily, it will just take a short period of introduction and they will be part of the flock in no time xx