Guinea pigs are more robust with their digestive systems than their bunny friends. However I still wouldn't overload your piggy with veg as it can still cause wet poo. Poor little Piggys can't produce their own vitamin C and so kale, spring green and broccoli will help your piggys keep up the Vitamin C in their body. Give your piggys kale, spring greens and broccoli once or twice a week. This allows the Vitamin C to build up in their body but without overloading them. Too much or too little Vit C can lead to wet poo. The quality of their coat is a good indication as to whether they need more.
Always check with a vet or online if a fruit or veg is toxic to the animal before feeding it. My advice stick to romance lettuce, broccoli, spring greens, kale, basil, parsley and very occasionally a tiny bit of carrot.