A single bird is the hardest integration. I think it might be better to separate a middle of the flock bird from your flock, and put her with the new one. They will squabble, but should settle down. Leave them together for another week. People who do chickens seriously call this a canary bird, as this bird can test the resistance of the flock to anything the new bird might be carrying. But in this case, I am suggesting that it is easier to introduce a pair of birds to an established flock.
Mrs K
Mrs K