If you post photos of the 14 week old we may be able to tell you the sex. One close-up of the head showing the comb and wattles whether it has wattles or not. Looking for size and color. A second photo in profile showing legs, posture, and conformation. At 14 weeks I'll ask for a third photo, sort of a closeup showing saddle and hackle feathers. A boy's saddle or hackle feathers can be sharp, a girl's more rounded. Not all of them have that at 14 weeks but many do and it is pretty definitive.
If it's a boy then puberty could be part of it, but it doesn't sound like it to me. It may be that puberty hasn't hit but it may be that it is an older girl. Or if it is a boy he doesn't see the others as girls yet. I get confused thinking about it.
You are trying to integrate a single chicken. That can be a little harder. They are flock animals and don't like to be alone. Often when an immature chicken invades the personal space of a more mature chicken it gets pecked. It's bad chicken etiquette for an inferior to invade a superior's personal space. It usually doesn't take long for the younger to learn to run away and avoid the older. When the single chicken is the older sometimes it wants to be with the others but the pecking instinct takes over when it invades their space. This creates an obvious conflict.
the "attacking" looks like pecking (pretty much anywhere) and pulling feathers, on one hen particularly.
This doesn't sound bad. If the pecking was concentrated on the head it would mean it was trying to kill the other. But this just sounds more like intimidation. "Stay out of my personal space" even if the aggressor is the one doing the invading. Sometimes they will kind of single one out. It might be a particularly weak chicken they can easily bully or it may be a strong one that they want to keep down so they don't lose their spot in the pecking order.
So what does all of this mean? My definition of a successful integration is that no on gets hurt. All that one big happy flock hanging out together can and will come later. If they avoid the older during the day and at night on the roosts life is good as long as no one is hurt. Typically my pullets hit that point where they merge with the adults about the time the pullets start laying. You are in a different situation with everything going on: a single chicken, not sure on sex, and that awkward adolescent (maybe) age. You did not mention that any were getting hurt, just that they run away. I think you are in a pretty good place with them.
