Hi!
No, a one-off vitamin C dose won't hurt (the excess will be excreted in the pee) but long term high dosages of vitamin C will ironically put your piggies at a much higher risk of developing scurvy symptoms because their body will get used to those high levels and will in turn react with scurvy (i.e. vitamin C deficiency) symptoms whenever that level drops for some reason even though the actual level is still well above normal levels.
Please be aware that vitamin C is already present in most foods that guinea pigs eat; in the greens, herbs and cavy pellets that replace the supplementing role of wild forage in their natural diet; fresh growing dog pee-free grass is very high in vitamin C and is the reason why guinea pigs never have the need to make their own in the first place and even good quality hay still contains a little when it is fed as the mainstay of a piggy diet. Recovery formulas and probiotic products are also enriched.
In the 15 years this forum has been running with literally several tens of thousands of piggies passing through here, we have hardly ever had scurvy issues. A few have come from private rescues of piggies with malnutrition but the rest have been overdosing cases. What we have not seen in either our own piggies or in forum piggies on a good hay/grass based general diet that they have ever had any issues with scurvy despite not being supplemented with vitamin C because they are quite obviously getting enough in a normal balanced diet and despite often living a health life span or even beyond.
Long Term Balanced General And Special Needs Guinea Pig Diets