You can feed your guinea pig with Grass Hay, Pellets, and fruits and veggies with more Vit.C and less Calcium. Something like Apple (but take off the seeds it is poisonous) and Lettuce. And more water intake.
When I had a guinea pig, I bought her food at the pet store. There were dried fruits, hay, nuts, and many other additives.
They eat Grass, pellets, and fruits.