Hi
@Eastmtmike 
Welcome To BYC
I have purchased 4 babies from a breeder; one 6 mths (Cochin bantam), 2 7wk old polish, and one 5mths astrolaup. They started coughing, sneezing, and have the smelly discharge from nose. Used the tylan 50 for the 5days (did once a day .23, .5, and 1mL for 5days and then learned should have been 2x day... tried that and they aren’t responding and starting to have symptoms at injection sites so no longer injecting). Been using vitamins, probios, and VetRX. Lost the 5mth old. The others still have all the symptoms.... at lost what to do next. The polish also developed weird wounds on their feet that I’ve been treating with vetracyn and Epsom salt. Completely lost as to what my next move is.
It does sound like they have a respiratory illness. Without testing, there is no way to know for sure what you are dealing with. Infectious Bronchitis, Mycoplasma, ILT and Infectious Coryza are some of the most common.
Do the best you can to keep them eating and drinking. Treat only the ones showing symptoms. Since they have all been housed together, whatever they have, the others have been exposed and would be considered carriers even if they never show signs of illness. If the ones that you have separated are lethargic and need extra care, then continue to keep them separate. Antibiotics can help with secondary infections, but won't cure the illness. Tylan50 can be used - you will need to weigh each bird you're going to treat. Injectable Tylan50 can be given orally or by injection - dosage is 1cc or ml per 5lbs of weight (.20 cc per 1lb). Give it 2x a day for 5days.
If you do happen to lose one, send the body to your state lab for testing will hopefully give you some answers as to what illness you are dealing with.
http://edis.ifas.ufl.edu/ps044