I currently have 8 chickens (mixed breeds) that I raised from chicks. They were vaccinated by the hatchery for Marek's disease. I am looking to add 2 more chickens to the flock. I have a friend willing to give me (2) 2-3 year old chickens since he is downsizing his flock. The chickens were never vaccinated for Marek's but appear healthy and happy. What concerns should I have with introducing these 2 new chickens?