My birds crave greens like crazy when going through a molt. Do you have any grass you can give them? Or will the local grocery store give you old wilty greens? My birds go nuts for collard greens, broccoli, and bok choy, and they even like green cabbage (didn't much like the red, though). Depending on where you live, there may also be edible weeds coming up. Mallows (cheeseweed), filaree (heronsbill/clocks), dandelions, and sometimes plantain are popular with our birds. We've had a mild winter so far, and the filaree and mallows are already eight inches tall.
I'm not sure if Sulmet would work for upper-respiratory illness. Much of the upper-respiratory stuff is caused by some very tough little bugs. Sulmet is, I believe, a sulfa antibiotic; it is labeled for some respiratory bugs such as infectious coryza. But it's not labeled for mycoplasmas, and mycoplasma is more common than coryza. On the flip side of the coin, with either disease you would definitely be seeing respiratory signs by now, and since you aren't, I think we can lay the respiratory thing to rest. Thank goodness!
Food-wise, I'd offer a good-quality layer feed, a little scratch, and add in some greens, especially if they aren't able to range due to weather or if there isn't much green out there. You can also try fermenting some layer feed and offering that as a supplement; I've been trying it out, and the chickens go ape for it. There's a good thread on fermented feed here:
https://www.backyardchickens.com/t/644300/fermenting-feed-for-meat-birds Yes, it talks about feeding meat birds and feeding only fermented feed, and that may be a little over the top. But I've found that offering the fermented stuff just twice a week has benefited our birds: they're molting more smoothly and not getting as worn out by the molt. Plus they really look forward to it. One hen tries to dive-bomb the bucket off my arm when I'm bringing it in!
