Reddit mentions of Probios Vet Plus Dispersible Digestive Powder, 240gm

Bactrim is fine for pigs, and is typically successful in treating URIs. Side effects are quite rare as it is a milder antibiotic, but it's possible that it could cause digestive upset. If you wanted to be on the safe side, you can give your pigs some probiotics an hour or two after their Bactrim dose. Probiotics aren't likely necessary but it won't hurt them if you want to do it anyway for your peace of mind. Even more rare side effects (but also more serious) are loss of appetite, severe diarrhea, and lethargy. Contact your vet if you notice those.

Because Bactrim is gentler it's a good 'first' option to treat infections, especially for young guinea pigs. There is a chance that it may not be effective if the infection is sufficiently advanced or simply antibiotic-resistant. If your pigs aren't starting to improve after 3 days, or if they ever worsen after beginning treatment, call your vet as they'll likely need a different antibiotic (usually Baytril, but not always).

Probiotic info:

Safe probiotics to feed guinea pigs are fresh poops from a healthy guinea pig if you can get it (this is believed to be the most effective, plus it's free ;D), or if not you can sprinkle some powder from an acidophilus capsule on their pellets, or a probiotic meant for small animals like BeneBac (comes in gel or powdered form, and is usually carried by pet stores and vets) or Probios powder. Pigs don't do well with dairy, so avoid yogurt or other dairy products.

Amazon links (if you live in the US): Benebac gel tubes and Probios powder (a big tub like this is overkill for your situation, but it is another option)

I'm certain there are plenty other probiotic options that will also work fine, these are just the ones I've used and know of off the top of my head. It's also a good thing to consider keeping on hand just in case.

Guinealynx page on probiotics

My dogs seem to get stinky when they eat foods with too much protein, so I've actually been feeding them high quality foods that only are about 26% protein (both my dogs are pretty inactive).

One of my dogs was getting a lot of seemingly spontaneous diarrhea, and so the vet had us try probios (a probiotic powder). He gets a scoop of it in his food in the morning, he's been on it for about 2 months now, and no diarrhea! We adopted him about 8 months ago and his stools never were really firm, and sometimes we could hear his tummy/intestines gurgling. We tried different foods, different tests at the vet, no luck until this. I was extremely skeptical but it really does seem to be helping, and it's cheap so why not. You can get the probiotic powder really cheap on amazon (it's almost twice as expensive from the vet here), I'll post the link below, but I do want to warn that the probiotic may increase your dog's appetite, so you may have to feed more than you have been. My dog got really grumpy shortly after starting it and we realized he was just much hungrier than usual. This is probably because my dog wasn't eating as much since he wasn't feeling well before, but it's something to keep in mind.

Good luck, I hope your pup feels better soon!

http://smile.amazon.com/dp/B001BM1QRC/ref=twister_B002HNOSYC