From what I've learned they should be the same size before putting them together.
You will want to introduce them through a fence for a few days or more, letting them see each other greet each other through the fence before actually putting them together.
Another way to introduce them is to put the new girls on the roost with the others at night when they are all sleeping. It seems to help if they all wake up together, but you will want to be around to watch and make sure everything goes smoothly.
I'm sure you'll get some other great ideas from more experienced people here.