Antibiotics will treat bacterial respiratory infections, like Chronic Respiratory Disease. However, some diseases like Infectious Bronchitis are caused by viruses, and viruses are not harmed by antibiotics.
I've read that Valbazen is the best dewormer if your birds have gapeworms. I believe that Safeguard can also work if you give it in a strong enough dosage twice.
I'm not so sure that your chickens have gapeworm. Gapeworm is relatively rare. You can tell for sure if they have gapeworm by...