I get sprouted greens at the grocery store for my indoor cats. They graze on them like small goats, and I'll bet guinea pigs would love them too.
And I won't feed my turtle iceberg lettuce because it is almost totally lacking in nutrition. If you give her any salad mixings at all, make them a little bit of dark, dark colored ones.
And as for the Vitamin C, remember that they are Guinea Pigs which were and probably are used as test subjects with human medicines and they are like humans, they can not make their own Vitamin C like animals do. They have to get it from foods.