In my *very* brief experience, free ranging helps a lot in introducing new chickens.
A few weeks ago we added a new pullet to a group who have been together since they were babies. They are all 13 weeks now. We kept the newbie in a separate cage when the chickens were in their run for a while, and then let them all free range in the yard together. Even though the others chased her, and tried to peck her, the new pullet (a barred rock) mostly just kept out of the other chickens' way. After a while we experimented with shooing her back into the run with the rest of them and they would bully and chase, but she found places to hide. Now they are coexisting, if not best friends. The original four are constantly together, but they tolerate the new girl.