Penicillin is a definite no-no - that much is for sure and that is the only thing that everyone agrees on. Some derivatives are okay - Griseofulvin is a penicillin-derivative and is fine to use on piggies (though an alternative, such as Intrafungol, would be preferred). Many antibiotics are safe, but the most common are:
- Baytril
- Septrin
- Doxycycline
- Chloramphenicol
- Flagyl
There's rarely a reason to deviate from that list. Most vets will stick with Baytril because it is the only one actually licensed for use in guinea pigs. Each drug is effective in their own ways - Septrin is good for UTIs, Chloramphenicol is good for the most stubborn of infections. Some can be given topically as well as orally and injected.
As you say, there is much disagreement between experienced keepers, vets and rodentologists globally on what is safe and not safe. In some cases, an antibiotic is given and the pig has subsequently died, and it is assumed that the antibiotic is not safe. The case could be that the dosage was too low, that the pig was too ill, that the antibiotic on its own was not effective enough - in each case the pig died of the illness, not the drug given to treat it.
As with humans, some pigs are intolerant of some drugs, which would worsen the condition and could lead to death. That is something else a lot of vets don't consider.
My experiences with piggie antibiotics were in the cases of pneumonia, cystitis and abscesses. I've been given Baytril, Septrin and Marbocyl for these issues - all safe and useful in their own ways.
I have also had one antibiotic injection given (Convenia nj) which was in a classification mentioned on the Guinea Lynx Dangerous Medications list. The pigs were fine, although I was concerned about the poops of one of them, but it was not a toxic medication, nor did it have harsh side-effects. Drugs listed on the Guinea Lynx Dangerous Medications list are not toxic in all instances, which is where so many people trip up e.g. some are dangerous if given orally, but are safe if given topically or injected.
It is amazing how something that should be so simple can bring about so many contradictions.
