I can share my own experience with you, but ultimately you have to make your own decision. Personally, I would never sprinkle anything on food and believe that my cavy actually ingests it. The only way to be sure is to syringe it to him.
Three or four months ago Sparky decided that he had no interest in his peppers, of any color. After about a week, I began to syringe the Vit. C two or three times a week. Then he had his episode where he almost died with CHF (congestive heart failure). Almost like a human who goes through a severely stressful experience, he began profusely shedding his hair in excessive amounts. Dr. Albin put him on two heart meds (Furosamide and Vetmedin) and recommended that I increase the dose of Vit. C. I increased the dose to 0.5 ml q.d. for the next two months. I just recently decreased the dosage to 0.3ml.
I don't know how much the Vit. C has to do with his recovery, but his fur is thick and shiny, and he sheds less than at any time in his adult life. And, he has a renewed interest in peppers. I buy the mini peppers in a variety pack of red, yellow, orange and slice them fresh for his salad. I never thought I'd see the day that he would choose the peppers to eat first from his salad, but that's exactly what he is doing these days. As an aside, I used to mix the drops with water, but now that he has other medication to take, I find that the less volume, the more amenable he is to taking his meds........and my boy takes them like a champ!
If the only thing that you are syringing is the Vit. C, then feel free to mix them with water or a little pedialyte. I don't recommend mixing anything with fruit juice, but then if that's the only way the girl will take them, then "by any means necessary".