I have my pigs' bag of Excell nuggets here (with added mint for digestion), it seems to be mainly fibre and an added probiotics with Vitamin C that is then important parts of pellets.
You can give them plenty of fibre (hay) You can give them their Vit C in their fresh veg (and some owners supplement their water with Vit C but it doesn;t stay active long in solution and you have no way of knowing if/how much they drink) Probiotic powders can be added to food, but again you can't control the intake
Too much veg isn't good for them either (some veg is too high in calcium, or too laxative)
My pigs don't finish their pellets but they do pick at them.
Is it the pellets themselves or would your DC be sensitive to the guineas pooh or saliva? If it's the pellets themselves could you feed them?
You'd need to make sure your DC aren't allergic to hay or you'd be struggling.
I love guineas, I started keeping pigs when I was 9yo. My DD started when she was 9yo with her lardy boars. I'd have to say there's a limit that you can change an animals lifestyle to suit the owner. You might find that guineas aren't a viable option for you A huge part of a guinea pigs health depends on their diet, and they are eat/pooh machines , it's their reason to live