It really sounds like you need a new vet. It just sounds like they're interested in your money. My vet generally uses Bactrim for respiratory infections. My piggies got 0.02 cc's by mouth twice a day for seven to 14 days. I'm not sure of the concentration, but I know they use the kid's stuff. Here's a list of rabbit vets. You should ask them if they see many guinea pigs also.
http://www.rabbit.org/care/vets.html and here's a page on how to find a good vet. It was written for rabbit owners, but it's similar enough.
http://www.rabbit.org/faq/sections/vet.html If Pinky's not eating, you can get Crita-Care, by Oxbow, from a vet. It's a special syringe feeding formula for small animals. If your vet doesn't have it in stock, the company will overnight it. The scratching could be mites, a skin infection