although probably not necessary we at home all of us have a supplement of vit C, also the piggies of course. I use drops for children for them, but I don't put the drops into their bottle. I mix 2 drops with 3 drops of water and I immediately give the mix to the piggie using a syringe. If you want to use a supplement, don't buy the effervescent tablets and the tablet with "stabilised" vit C (for example the tablets from Oxbow), because it is not sure if the light, storage and air have or not oxidised part of that vit C. Drops are better (and cheaper) and you can dose them more accurately.
Consider also the amount of vit C of food and pellets, which are also fortified.
The danger for kidneys and joints is a huge amount of excessive vit C, for example 5000-9000mg a day, but one drop of the supplement I am using is only 6mg. A little amount is protective, a huge amount makes damages (also in us humans).