If you have pro-C, then by all means give that. It is a vitamin C enriched probiotic.
The 1/8 of a tablet is in response to somebody who cannot get hold of any piggy specific products and needs to know how much of a human tablet will do in a pinch. It is a less than ideal solution but keep in mind that we have enquiries from all corners of the world and need to find some kind of solution for members that can't just go to the next pet shop or order from amazon or ebay.
You will find advice like this in several guides as we need to give those members options that work anywhere in the world even if they are not strictly the best. It is not relevent for you as you have access to UK brands.
If your piggy is not eating as much and is on medication anyway, please switch from weighing once weekly to weighing daily at the same time in the feeding cycle like always first thing in the morning or after dinner as the difference between a full and empty bladder and belly can make up to 40g. If your piggy is losing weight, then please step in with support feeding that is appropriate to the severity of the problem (i.e. offering some extra mushed pellets for a minor issue, adding some extra syringe feed 2-3 times a day for a guinea pig that slowly losing weight or stepping in fully with round the clock syringe feeding support if they aren't eating at all or only nibbling very little. Always keep in mind that you cannot control the hay intake by eye - and that is 80% of what a piggy eats in a day. Veg and pellets account for just about a fifth of the daily food intake - basically your daily snacks.
In guinea pigs with respiratory illness, you have to be aware that the need to breathe comes before the need to drink and thirdly the need to eat. This means that if breathing is difficult, your piggy will lose their appetite. Add to that that antibiotics don'y just work on the bacteria in the respiratory tract that are causing the illness but often also impacts on the gut biome that is responsible for the digestive process and the nutrient absorption, then they are facing a bit of a double whammy until after the end of the course of antibiotics.
Please take the time to read these guides here:
Weight - Monitoring and Management
Complete Syringe Feeding Guide
I hope that this helps you?