Place your lowest ranking chicken in with your new chick for a while then gradually add another etc. Do not add it to the flocks original pen for awhile until it and them are use to each other in the new pen where the little one is. It will help some. There will still be some pecking going on. The pecking order is usually established by about 6 weeks.
It's the lowest ranking one that seems to be so aggressive. I know that since our roo just left that they are re-establishing the order but I'm worried that I've made a huge mistake with getting the baby.