Most chickens will peck or fight a little to establish a pecking order within a flock and this is common. If you add new birds to an established pen of chickens they will fight or peck a little to establish a new pecking order and it is recommended to keep a watch on them for the first 1-2 days. To reduce the tendency of them wanting to fight as much it is best to put all the chickens into a new pen that none of them have been in before.