Do not add a single bird! You can do it, but more than likely it will not be pretty. The more young birds you add, the better. A single new bird, all of the established birds will get their licks in. If there are several, the home team will have several to pick on, and it spreads it out.
Measure your space. Making sure you have enough space. This time of year, free ranging will not help, as the birds are roosted for most of the 24 hours. Provide hideouts, extra feed bowls, and water. Watch but don't interfere unless there is blood. If there is a single very aggressive bird, you can pull her out for a couple of days.
Chicken society of a flock is closed, and it can be a while before they will become one flock. Probably when the pullets start laying. Until then they will be a sub flock.
good luck,
Mrs K