The Duramycin is good. Make sure you use it for the full number of days as it says on the label. Nobody is laying eggs yet, so you don't have to worry about tossing the eggs following treatment. Make sure you mix a new batch every 24 hours, because the mix becomes unstable.
A good practice when adding ANY new birds to your flock is to quarantine the newcomers for 30 days. Even if the newcomers don't look sick, they could still be carriers of a disease (maybe immunized as chicks). Anyone in your flock who doesn't have immunity could get sick and die.