I've used all types of bedding and hay, and my guinea pig has never gotten mites. I would get a second opinion from another vet that's not associated with your current vet. I wonder if your guinea pig could have gotten the mites from your cat, since it goes outside everyday? My understanding is that mites can't live in bedding or hay that's on a store shelf, because they have to have a living host. There's a lot of information on the internet, so it's sometimes hard to figure out what is and isn't the truth.