Some pecking and bullying is normal when introducing new members to an established flock. The flock's dynamics change and new members will be need to find (or rather be put in) their place in the pecking order. This could take some time, depending on the flock. When introducing new chickens it's better to keep them separate, but within sight of the existing flock, at first, if possible. Especially chicks and younger chickens, who are not fully grown yet. After that let them free range together, if you let your chickens free range. Give the newbies enough space to get away and hide from the older flock members, if they need to. Also keep an eye on them at feeding time, make sure the newbies are not getting chased away from the food and put some extra feeders and waterers in the run if needed, to make sure the newbies can eat and drink enough.
