Just thought I'd share my experience. I acquired chicks this spring there were carrying something respiratory as well. I was unaware of this until two of my grown hens in the same coop with the broody and her (adopted) babies started exhibiting terrible respiratory symptoms. It was bad enough that I didn't think they would make it. After 5 days on duramycin the hens were completely symptom-free. However, because I didn't know where it originated from, everyone else had already been exposed. Over the next week or two it gradually made it's way through my hens (who only displayed minor symptoms, besides the two original symptomatic ones), and then through my chicks where it really took it's toll. It only took 2 of my 18 chicks, but everyone was sneezy, stuffy, and congested and did NOT respond to the duramycin. Now, 3 weeks later, the symptoms have all but been eradicated. Their immune systems have taken care of it except for some residual sneezing.
I am currently treating everyone with Denagard for 5 days to see if that stops the sneezing (it was supposed to arrive two weeks ago...but my package was lost). If it doesn't, I'm assuming it's viral and will have to depend on their immune systems to take care of it. I refused to cull because my flock is closed and these birds are pets, and because apparently it's been going around the area anyway (I read somewhere that a huge percentage of flocks have been exposed to MG or some other CRD causing microbe...but that not all birds are symptomatic throughout their life).
For you I would skip the Duramycin altogether and get some Tylan powder or injectible. The birds in question are older, correct? If so, order some injectible Tylan, give it one shot in the breast, and the symptoms should disappear. However, introducing them to your flock WILL cause the others to become infected. The choice is up to you whether or not you're willing to manage a CRD flock or not, meaning you cannot sell hatching eggs, chicks, or birds in the future. Chances are you have not been showering between coops, so your other birds may already be exposed as the pathogens can travel on your skin, shoes, clothing, and hair. Incubation periods are usually 5-14 days, so keep a lookout for symptoms in your other chickens.
Good luck, only you can make the best decision for you and your flock.