Vitamin C is water soulable, but overdosing can lead to serious problems, it does build up in the system however it is rare to reach dangerous levels, then again I don't know what level a guinea pig can take,
they can handle plenty of Vit. C, remeber Guinea Pigs I believe are the only mammal other then humans that cannot produce their own Vit. C,so it has to be supplemented in the first place lol

I am glad Domino is doing better, it is very likley that he got a cold from you guys, as I am sure you know they they do catch colds and other illnesses from people.