URI/Upper Respiratory Infection Guinea Pig URI treatment at home?

Hi! I have two guinea pigs that I got some months ago from Petco (I know that was my first mistake, I promise that now that I know some rescues in my state, if I ever get another (2) guinea pigs, they will be adopted from a rescue, rather than bought from a pet store.) When I got them they were still young and smaller than they are now. I got one healthy guinea pig (it was alone at the store), and then a few weeks later got her a playmate once the store got more female piggies. (I searched those two weeks at various Petco's and Petsmarts, but they all were either out of stock, had only males, or had females that were a lot bigger than mine, and I read I should get one that's around her size & age to have a likelier chance of them getting along). Once I got the second guinea pig, I realized after a couple days that she was sneezing and had goopy eyes that stuck closed. My other guinea pig started sneezing too, so I took them both back to Petco to get healed (because they give a 30-day health guarantee in which you can bring them back if they get sick). They were getting treated at Petco for a good two months, first for the URI's, then one was getting treated for Bumblefoot, then they were both getting treated for Ringworm. A few days ago I finally got to bring home the second guinea pig I adopted, but the other one is still being watched to make sure the Ringworm has gone. I cleaned and disinfected her cage, her igloo, and her food bowl. I also got her a new, larger water bottle, just because the older one was small and got emptied in a day (I didn't want to risk having them dehydrate during the night if they emptied their bottle when I go to sleep and go through the night thirsty). This is where I think I made a second mistake. I'm not 100% sure how URI's can be transmitted, because I know they can be contagious from pig to pig, through sneezing. I don't know if they can also be transmitted through items they have both touched, and/or water bottles they have drunken out of. I realized my piggy hadn't tried drinking water from the larger, new water bottle so I got worried that it was too big and she didn't know how to use it, so I put the old water bottle back in. It was filled with new water of course, but it wasn't cleaned or disinfected. She drank out of it once, and then a while after I saw her drink from the bigger one, so I took the small one back out. Thinking of it now, there's also a corner litter-tray that I kept in the cage, and i'm positive the bowl part was disinfected when I redid the cage, because I like to clean off all the waste possible, but I don't know if the wall part was disinfected, if that makes sense?

Anyway, my guinea pig has started sneezing almost only a day after I brought her back. I thought I was paranoid at first, but she keeps sneezing and I don't want to pass it off. She did come with another 30-day guarantee but i'm hesitant to bring her back to Petco only a few days later. They sold me a sick piggie, and then she got treated and I brought her back and I think she's sick again. I don't want this to just become a cycle. I looked up what the treatment for guinea pigs with URI's is, and I saw that the most common were Bactrum or Baytril. I've seen good reviews about Bene-Bac (which came up for Bactrum, so it's the same, right?) in a gel form that you syringe orally. Can I use that to treat URI's? I would like to get her healthy in my own home, so there's no bringing-her-back-home-and-her-getting-sick-again situation. I'm more than willing to buy Bene-Bac, and treat her myself. My concern is, would this really heal her? How much should I give her and how often, and for how long? Is there another medicine that's better, or a medicine that I have to give along with it? Any other advice, precautions, or something that i'm doing wrong? She deserves a much longer and healthier life than this, and i'm not going to let her die without doing everything I can. If my idea sounds wrong, I will send her back to Petco to get healed again and do a much better job of replacing everything in the cage that they have infected before. But if I can do it this way, I would really appreciate the advice! Thank you <3