My guinea pigs consume their Vitamin-C drops daily, sprinkled on a plate of clean, organic, torn lettuce. I probably sprinkle 18 drops or so on top and they eat it up readily. I don’t add it to their water. Mine didn’t like it when I tried that 5 years ago. They need it daily. Their fur gets nice and shiny when they get those Oasis Vitamin -C drops daily. My guinea pigs have lived longer since I discovered those drops. This is my experience, and other people may have different opinions. One of my guinea pigs is 6.5 years old. 

Vitamins don’t expire like antibiotics do. They become less potent over time. If you are giving your pet clean water daily that bottle will be gone long before the potency of the vitamins begin to decrease. I generally buy 4 bottles at a time (we have 5 guinea pigs). Hope this helps

I use Vitadrops with the green lettering .  It’s a multivitamin with extra vitamin C.  I use it temporarily when the vitamin C is out of stock.

No, vitamin C does not affect hair, nails, and hair loss. However, vitamin C is essential for guinea pigs as they can't produce it on their own. I do not suggest using drops because they dilute within a few hours and make the water taste funny. I instead suggest using Oxbow vitamin C tablets as they are the best kind you can get. For hair loss, ask vet because it could be a much deeper problem.

definitely yes

no, just like humans they urinate the vitamin C that the body doesn't need.