I think the sulpha drugs will only help if they also have coccidiosis. Since coccidiosis is common in chicks that also have immune problems because of a respiratory or other disease, the sulpha medication can give them a boost and help them through the virus/germ. But they basically still have to get through the respiratory disease themselves, especially if it's viral.
Remember, without seeing the chicks I can't really say whether they have coccidiosis, a respiratory illness alone, or a combination... I'm just presuming going on past experience that sick chicks usually have cocci (whether alone or on top of another illness) and so giving them treatment for cocci may help get them eating properly and therefore able to fight the other ailment. (The first sign of cocci is going off feed, and when that happens other illnesses can be opportunistic.)
It's a shame a chick was sneezing on arrival, that's always a worry. We've all done that! It's still best just to focus on good husbandry and giving the chicks the best chance to get over it... The owner of the chicks you brought in probably didn't know they had an illness, and stress of moving them probably brought it out... Very common!
It may still be a good idea to separate all affected birds and any new sneezers... If you remove all birds as soon as they start sneezing then you may stop it spreading. But at some point you would then have to decide whether to put recovered sick birds back with the main flock (when some may be carriers), so that's why I don't say this is the only way to go...
Personally I would probably have a go at separating the sick ones, maybe keeping all chicks together to make sulpha treatment easier, and try just basic TLC after the sulpha treatment. It may be that you keep having losses... I hope not, but if the sick ones are always taken away from the others then at least you may stop it spreading through all the birds. Later (say in a week) you might have a better idea of what the disease is and whether it's serious in terms of the whole flock.
I'm sorry I can't say if the sulpha medication will cure or just help... It can't really cure a respiratory ailment (it's not a proper antibiotic but I gather it does have some antibiotic properties) but it may allow the chicks to feel well enough to eat and therefore survive better.
Hope that makes sense. (Again, I'm no expert... Just thinking it through in terms of what I'd do if it happened here... As it has in the past.)