Unless your piggy is going off its food or developing soft poos/diarrhea, probiotics are not necessary, to be honest. Guinea pigs of mine have recovered just as well with and without them in my own experience, especially in the olden days before there were any probiotics!
Septrin is much milder on the gut flora than baytril and doesn't usually cause any problems, even though we get the very occasional case of a bad reaction to any antibiotic. We usually recommend probiotics with baytril, in order to help bolster its effect as much as possible, as it is still the most widely used and only officially licensed antibiotic for guinea pigs.
If your piggy is really experiencing gut problems, it is often more effective to syringe poo soup (water in which you have soaked fresh poos from a healthy guinea pig). It mimics what recovering guinea pigs do, i.e. digging in their comrades' bums for caecotrophs to re-stock their own guts with the right bacteria etc. This is a bit gross for many people, who prefer giving probiotics.
Fibreplex has its place and can really help if you have got a guinea pig that is experiencing ongoing digestive problems with bloating, diarrhea or gut stasis, or that is slowly recovering from a major illness with a massive weight loss/reduced food intake over a longer period. It can also help with boars with regular impaction. But you are better off ordering it freshly unless you have got a number of frail guinea pigs to need it regularly.
