I can't answer your question. I suggest you call your county extension agent (in the phone book under county government) and talk to them about it. They should be able to give you some accurate information. I can pass on some information, but this is not a comprehensive answer to your question.
Many people keep turkeys and chickens together because of Marek's. When chicks are vaccinated for Marek's, they use Turkey Marek's to vaccinate the chicks. Turkey Marek's does not prevent the chickens from getting Marek's but it does prevent the lesions that cause the problems. So do not worry about keeping chickens and turkeys together because of Marek's. It is a non-issue.
Blackhead is deadly to turkeys. Chickens are a lot more resistant to blackhead. If you have blackhead in your area, it is common for chickens to house the disease and give it to turkeys and kill them. If the chickens were not present, the blackhead disease would die out and you could soon get new turkeys. If you have blackhead in your area, this can be a problem. If you do not have blackhead in your area, this is not a problem. Many, many people do not have a problem with this, but some do. It is a legitimate question to ask your county extension agent.