Cobrien does have a good point. I think I would agree with him if these birds were of "unknown provenance". Besides the disease he mentioned, there are viruses that can lay dormant for a long time (kind of like the human herpes virus), and be activated by stress (like being dumped in a strange flock, in a strange location). Just like the herpes virus, they would not be very contagious until they were activated. So in that case moving them away from the flock could still be very helpful. In this case it sounds like you have established that these are hatchery birds, that have not been exposed to other flocks (is this correct?), so I think you would be okay.