First let's address your "treat" question. My cavies have never cared for commercially produced treats........and I don't like them either. Better to offer those special things like corn husks, radicchio, or a baby carrot as a treat. Others here do buy healthy treats and relate that their cavies enjoy them. On rare occasions I will buy something upon recommendation, and invariably have to give it away because my piggies have no interest.
I highly recommend
Oasis Vita-Drops Pure C which you can find in almost any pet store or on Amazon. My boy occasionally goes on strike and won't eat peppers so a good source of Vit. C is essential. Forget adding these drops to water! DON'T DO IT! They change the taste of the water and the cavies won't drink it......which leads to other issues. And, I don't recommend adding them to food in any way. When you add anything to their food, you have no way of knowing how much (if any) that they are ingesting. Always syringe anything liquid.
I agree that 250mg is excessive for a cavy. 25-100mg is adequate under almost any situation. Here are the recommendations for the Oasis drops:
Normal healthy cavies - 25mg (8 drops) daily - about 0.2ml
Pregnant or nursing - 37.5mg (12 drops) daily - about 0.3ml
Showing symptoms of scurvy 50-100mg (16-32 drops) daily - about 0.4+ml
When my boy was ill and losing his hair, I syringed 0.5ml/day. Now that he has recovered, I have cut that down to 0.2ml. I generally syringe it full-strength, but you might want to draw up 0.2ml and add an equivalent amount of water. It's pretty sour, but he seems to like it anyway.