I have to boars who LOVE the pills! If I just shake the bottle, they go completely nuts. In the past however, I had cavies that didn't like the pills AT ALL. I give 1 pill to both every other day. It is a 60 mg pill without sugar. I do it because I know they love it, and because 60 mg can't be an overdose (in the way that you give so much vit. C that the vitamins will get stored, because the kidneys can't handle it anymore). I guess that the overdose number of vit. C even for cavies lies very, very high.
For the rest a high quality cavy food (WITH added vit. C) and fresh vegetables should be enough to suplly enough vitamin C.
A full grown cavy need appromately 20 mg of vitamin C a day. A young one 3 times as much.
Some high-end vitamin C suppliers are:
Broccoli (113 mg/100 grams)
sweet pepper (100-150mg/100 grams) (depends on colour)
parsley (172mg/100 grams)
pink rosehip (500mg/100 grams)
sprouts (150mg/100 grams)
chicory (114mg/100 grams)
Not-as-high-as-you-think:
cucumbre: 10mg/100 grams. Cucumbre is mostly water.
Carrot: 5mg/100 grams. Still good for vitamin A
Apple: 10-12mg/100 grams. Not that high eh?
Pear: 4mg/100 grams.
Endive (is this the right word???): 10mg/100 grams.
Hope this is a bit helpful.