if you are going to buy a commercial diet you might as well buy the one specifically made for guinea pigs as it will provide the necessary vitamins and minerals that guinea pigs need and might not be gotten anywhere else.
Cavies are not necessarily rodents, there is still a lot of debate that they should be classified in the category of rodent, although most feel they are pretty distinct as far as rodents go. They are not like mice and rats, they have different proteins in their blood, they don't give birth to hairless helpless babies, they cannot produce vit. C in their bodies and do have different dietary requirements.