To be honest w/ you....I got my first guinea pig at 12. I never read a book or did anything 'special' for them, just fed them guinea pig food, and those two lived a good long while.
Then - I got another 2 guinea pigs when I was 20.
Because we also had rabbits (and b/c I didn't know any better - this was 20 yrs ago -

before the internet) we fed them rabbit food b/c it was cheaper than guinea pig food and we already had that on hand....it LOOKED the same.
My favorite one got sick and died after a year, and I felt so bad I took the other one to the pet store and 'gave' it to them, because it wasn't 'right' either....they explained that they NEEDED different feed than rabbits, and that's what killed her, more than likely, a vitamin C deficiency.
Soooo....when I got some again years later.....I added the vit. c to their water probably more to salve my guilty conscience over the last batch than anything, but you can bet your bippy they got fed GUINEA PIG FOOD after that. They seemed to be drinking their vitamin water just fine, but then again the pet shop I bought them from used it in their waterers so they'd been 'raised' on it.
I've cut branches from apple trees for them before, and I buy the little flavored wood blocks and put them on the cavy, prairie dog, and rabbit 'stack toys' in their cages.
Not sure about peach or pear tree wood, sorry.