I am 40 and have had guinea pigs since I was about 12. When we first started out, I used the guinea pig pellets as main diet and my guinea pig had a vitamen deficency. Our vet recommended Cheerios and all my guinea pigs have been on that ever since with no issues at all and lived VERY long lives (my friends used to comment about how they are "STILL alive after all this time!?".
Also, for care requirements, make sure guinea pig has hard things to chew on to keep teeth filed down and you need to make sure claws are trimmed regularly.