When you introduce chickens, you can't just throw them in together and expect them to get along. You should always set them up where they share a portion of fence. Keep them side-by-side for a few weeks where they can look, chat, interact, but not touch.
Then try putting them together. There will always be some chasing/pecking/sparring when adding new chickens. That's how they establish their pecking order. However, once that settles, there should be no more harassing. It may take several attempts before they're all okay together.