I just posted about quarantine-look at the Sticky at the top of the Diseases & Cures. I put Hawkeye in a large wire dog kennel in my bsmt bathroom, dusted him and his bedding with DE, fed him proper food (he'd only been fed corn), yogurt, vitamins, handled him and watched him. A disease can take up to a month to show itself. At one point I would take him up to my deck where I had another dog kennel and he'd get to bask in the sun eight feet above my girls' heads, so no contact. I then put him in a pen next to theirs so they could see him. A couple of the Wyandottes did flare up at him and grabbed his huge comb through the fence, but when I finally let him in the main pen, and put the girls in one by one, he worked it out by putting them in their places immediately. After that day, they were one big happy flock with a new leader.
A smaller rooster may not mess up their feathers quite as much, but he will still grab them by their neck or head.