It is best to integrate new chickens gradually. If you just put them together, bullying is virtually inevitable, and it may get better, or it may get worse and feather picking may develop. Once feather picking starts, it does not get better without some intervention (e.g., giving them more free range time, separating them until feathers grow back and then reintroducing them gradually).
Also, it is easier to introduce more than one chicken at a time that know each other. Much easier on the newbie if it has at least one friend. Also harder for the resident chickens to focus all their aggression on one bird - it can be sort of spread out among the new ones, and then can die out. The smaller the resident flock and the larger the new flock, the easier it is to integrate (assuming you have adequate space for all the chickens)
At least that has been my experience. I would separate now and try gradual reintroduction.