I'd say a bit of porphyrin staining around the cage is normal. Given that porphyrin is produced in their nose as well as their eyes, and everybody sneezes from time to time, anything that's in their nose is going to get sneezed out, so you're going to get some porphyrin splattering. It's only when the sneezing or porphyrin is excessive that there's a problem; how much is "excessive" is hard to say unfortunately. If it's only occasional sneezing, there's not porphyrin everywhere, they're not wheezing/rattling, then they're probably fine.
Some rats are especially prone to upper respiratory issues and will be a little sneezy and sniffly all their life. I've got a couple of rats like this, both of them rescues (descendants of mis-sexed PAH rats
) and I've decided to only medicate them when it sounds like the respiratory issues are starting to get out of hand. If it's just the occasional sneeze, snuffle, or rattle, I leave them be, and only treat if the symptoms are getting worse. Otherwise they would have been on antibiotics their entire life, which would probably have done more harm than good in the long run.
What dosages of the various medications were they prescribed? It is unfortunately not uncommon for vets to underdose medications in rats. For a mycoplasma flare-up, which is what this is most likely to be, a combination of antibiotics is often recommended: 10mg/kg Baytril twice daily, and 5mg/kg doxycycline twice daily, for two weeks.
Bedding and hay are common triggers of respiratory symptoms. Some beddings/hays are generally dustier than others and more likely to be culprits, but reactions are also rather individual. (Aubiose and Readigrass both set me off but my rats were fine, including the sneezy ones.) Back 2 Nature is generally very well-tolerated, but it's possible (though unlikely) that your rats find it irritatingl. It is however expensive if you're using it as a general substrate and not just in the litter trays; I'd recommend a cheaper substrate that you can buy in bulk for the main cage tray, such as chopped cardboard.
PAH nuggets are generally considered poor quality, and quite boring to eat as well. I'd recommend heading over to the Rat Rations website, where there are a variety of much better quality and much more varied diets available: http://www.ratrations.com/complete-mixtures-for-rats-c-54.html?zenid=p8h579ksv0pc3mkfmpchvlksi5
If you go with a Rat Rations mix, you'll need to add a bit of vitamin and mineral supplementation yourself (commercial mixes have it sprayed on already). I like DailyRat3 as it's most economical, and if you're feeding fresh veg you can stir it in to that: http://www.ratrations.com/vitamins-supplements-treatments-supplements-c-110_112.html
They also sell substrates, which can help spread the cost of the postage: http://www.ratrations.com/bedding-and-substrate-c-60.html