Hello Claire, and thanks for your question.
The following will provide a good guide as to what to feed your guinea pigs:
"Guinea pigs are strict herbivores. They should be fed a complete, pellet diet made especially for guinea pigs that contains at least 16% crude protein. The pellets should not be fed more than 90 days after their milling (check the bag or box for the milling date). Also provide small amounts of grass hay, and supplement Piggy's diet with a source of active ascorbic acid, such as a handful of cabbage or half a handful of kale (washed and fresh) or a quarter of an orange. Because guinea pigs can't produce their own Vitamin C, you should add a Vitamin C supplement to their water-a teaspoon of Vitamin C liquid to 12 ounces of water. The water will need to be replaced daily, however, as the Vitamin C will lose its potency rather quickly.
In addition to the above, the following fruits and vegetables-fresh, washed, and with seeds or pits removed-can be fed as treats:
You can find more information on how to care for your piggies here, the source of the above info:
As you can see, piggies do need vitamin C, as they don't manufacture themselves, and a supplement is always recommended to be added to their water.
The following site will provide more important information re: your guinea pigs, and it IS recommended to neuter males. Make sure that you do have two males and not one of each gender, as you would not want them to reproduce.
Find a veterinarian in your area who sees 'pocket pets', and has experience with guinea pigs.
I wish you much good luck with your furry companions!