That vaccine is fine if you have MG in your flock. It won't do anything for coryza. I feel bad for you having some rare birds, but unfortunately this happens to a lot of people. Many times people will sell birds from flocks that have been coryza or MG positive, and of course even birds showing no symptoms will be carriers. Keeping new birds quarantined for at least 30 days sometimes will prevent you from bringing in a disease, but not if they are carriers that don't show symptoms. MG is easier to treat and to vaccinate for, but you really need to get a bird tested. Your vet could probably do that with a blood test or nasal swab to send in for testing. If you read the edis link I gave in post 2, I think coryza is one disease that only affects chickens. I'm not sure about your other birds carrying it to new chickens--I would be tempted to guess "yes," but your state vet could tell you over the phone as well as advise you. Sorry about your predicament.
Mycoplasma or CRD can transfer to hatching eggs, so unless you get your sick chickens tested, I would not hatch chicks. I suppose people eat chickens with diseases that are not on antibiotics, but I probably wouldn't.
