On the subject of my mareks vaccine experiment, I have found that I can vaccinate them at any age and the vaccine works. I keep them separated until a week after I vaccinate them and this seems to be successful. The birds that I have vaccinated have been growing faster and healthier than the unvaccinated birds. I have had fewer losses due to coccidiosis or URI's and the vaccinated birds are a ful 1/3 bigger than the unvaccinated birds, this includes my silkies, ameraucanas and meat birds.
I have been reading studies that have suggested that vaccinating birds at any age can be effacacious, though this is off-label. I would like to organize a mareks vaccine clinic (ala rabies clinics) for my county, but I'm not sure if this would even be legal.