If you're already having problems with your adult chickens picking at each other and feather picking, I would not add any more to that flock. Those are signs of over crowding, and adding more chickens will only add to the problem. You will need to expand your coop, run, or wherever you keep them. Do you already have younger chicks? If you don't, I would suggest not getting any more until you get this problem straightened out. If you already have them, you should probably keep them separated until you have more room. Now let's move on and say you have improved the conditions your current chickens are living in, and they have lots and lots of space so you can add new ones. There will be squabbles to establish pecking order. That happens no matter how much space you have. The younger chicks should not be added until they are close to the same size as the adults and you have provided places where they can get away from the older hens. It would also be helpful if you have at least two feeding and watering stations so the older ones can't keep the younger from eating and drinking. Chicken relationships are not always pretty. They can get nasty while working things out between two different flocks. But I will repeat myself - you SHOULD NOT add more chickens until you get your original flock to the point where they are not feather picking and cannibalizing one another.