Hi
As you are in the UK, you have to be aware that every vet is referring to a medications database that lists the species-specific possible dosage range band from the lowest to the strongest. This is in order to prevent overdosing. You can rest assured that while your vet has maxed out the possible dosage range, they are not overdosing when battling a non-standard bacterial infection.
You cannot compare those dosages with the kind of standard problems that our daily bread on here. If you have anxiety issues, then doing DIY online research can be really a problem. 'I think my piggies have been killed by overdosing' is not exactly proof of anything but an owner trying to make sense of a bad reaction to baytril - something that can happen with any antibiotic.
Other countries may not necessarily have this centralised medications database available for guinea pigs, so in those places overdosing may happen. That is another aspect you will have to take into consideration; which part of the world those posts are coming from.
A third aspect to take into consideration is that every antibiotic covers only a slice in a whole wide spectrum. Your vet is trained to know which antibiotics are best for which type of bacteria. Guinea pigs and other rodents have a more limited range of antibiotics available due to a rather different metabolism. If there is no ideal antibiotic available for guinea pigs, then the next best alternative is use the antibiotic that comes closest and to max out the dosage.
What you have brace for is that the higher the dosage, the more likely baytril is going to knock out the gut microbiome. It is one of the antibiotics that is more prone to do so.
Please be very generous with prebiotics and get hold of recovery formula for syringe feeding two piggies who will likely lose their appetite completely.
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Probiotics & Live Gut Microbiome Transfer ('Poo Soup'); Recovery Formula Foods And Vitamin C: Overview With Product Links and Transfer Recipe
If you are not happy with your vet's prescription, you can ask for a referral to another vet.
I am very sorry that you are up against a rather tricky foe where there is no easy cure but I hope that my detailed answer is helping you to understand your vet's decisions a bit better.