
Having kept a large stud of 300 longhairs, and breeding, showing and judging for 25 plus years, I have experimented with most foods for them.
The best ever dry feed, that had everything they needed was not commercial guinea pig feed. I always fed Spillers coarse horse from the local farm merchants. I am fairly certain that the Cambridge Cavy Trust did loads of research, and I got the tip from them.
Vitamin C was a biggie, and very important if you wanted good skin - which you needed for exhibition. Mine loved cauli leaves, but, having so many, I used to buy crates of spring greens, cabbage etc. I never, ever saw a pig with the runs from feeding greens. It is however, important to note that you should never feed yellowing greens. It is impossible for a cavy to vomit - so if food isn't fresh enough for yourself, don't feed it to your pigs.
Apples and carrots were fed freely, and if I wanted to put extra condition on the pigs, I bought sacks of beetroot. Cut it into quarters, as they won't eat the skin. I always lined cages with thick newspaper, as it made it so easy to lift everything out when cleaning out (used to take me 8 hours!)
I used clean white wood shavings (never, ever sawdust) and heavily covered with sweet meadow hay. As I fed this in huge quantities, they trampled down what was not eaten, and it made a better bedding for them. Always give large quantities of hay, it is extremely important to their digestive systems. And empty stale water out of their bottles every day, and give fresh. It's important to know that they blow back up the spout on their water bottles. They can get totally blocked. I used to soak spouts in hot water weekly, and bought pipe cleaners for cleaning out the muck from the spouts. If you get them trained, you can let them out in the house, and use a litter tray. Poos are no big deal, as they are totally dry and can be picked up easily.
I must have been doing something right, as I expected mine to live 9 - 11 years, which surprises people. The people who didn't feed enough hay! :-)