From a flock management standpoint this is a good idea, however you will eventually run into a genetic bottleneck and the "family tree" will start looking more like a twig. You have done the best you can do with the situation that you were given...
Does this sound like the symptoms?
Infectious Coryza Incubation Period hours to days Course time is days 3 months
An acute to chronic infectious respiratory disease of chickens, pheasants and guineas caused by a bacterium. Chickens 14 weeks of age and older are most susceptible and it increases with age. Characterized by conjunctivitis;, catarrhal inflammation of the mucous membrane of the upper respiratory tract (nostrils, sinuses, and upper windpipe); sneezing, swelling of the face and reduced egg production in hens. Transmission is bird to bird within a flock. Recovered birds remain carriers. Within an outbreak flock, contaminated feed and or water is probably the mode of spread. Birds risk exposure at poultry shows, bird swaps and live bird sales. Clinical signs are swelling and puffiness around the face and wattles, thick stick discharge from the nostril (and a very offensive odor!) , labored breathing, and rales are the common clinical signs. There is a drop off in in feed and water consumption as well as egg production. The birds may have diarrhea and growing birds become stunted. Illness persists for several weeks, continuously if complicated with mycoplasma gallisepticum. Treatment is usually sulfadimethoxine (Albon). If Albon fails or is not available, sulfamethazine, sulfamerazine or erythroymcin (Gallimycin) can be used as alternate treatments. If you have an outbreak, segregate birds by age and dispose of dead bird by incineration. All replacement birds on a "coryza endemic" farm should be vaccinated.
I would also look into good biosecurity practices... the NH Coop. Ext has some good info on this:
http://extension.unh.edu/Agric/AGDLEP/PoulRab.htm
Don't give up on adding new birds, as needed, just be careful with who and where you buy from...
