As a long term keeper of guinea-pigs ( sadly now all across the Bridge but not from Veg Overdose ) I would say be guided by your guinea-pigs . They are experts at being guinea-pigs 
First off - your pigs should have unlimited hay . At least a wedge of hay their own body size but more is better . They eat , play , sleep and pee in it .
Pellets not muesli
Fresh water that they will likely ignore .
They will get their Vit C from pellets and veg so you don't need Vit C in the water unless your piggie is ill . It can affect the taste of water and they don't like it .
Give a variety of veg .
Limit fruit , it is too high in sugar to be given regularly - can lead to diabetes in pigs , and can ferment in their gut causing yeast imbalance .
Carrot , swede, parsnip all high in sugar too
Dark green veg ( brocilli, parsely , kale , watercress ,cabbage ) are high in oxilates which can make them prone to bladder stones
Don't give iceberg but other lettuce is ok.
Slices of pepper ( loved red/yellow/orange didn't like green)
Ours loved sweetcorn in leaf, parsley , leafy bits of celery , leafy carrot,
Grass and dandelions when they were grazing , hand cut when they weren't .
We gave ours maybe 3-4 varities each meal , I used a metal dog plate for 3 adults (about 8" wide but shallow)
Other treats - watermelon rind with a bit of the flesh , banana with skin, strawberry leaves , cutted up grape , blueberries , tiny slice of apple .
Pumpkin and orange segments were spat out
Lambs lettuce was scorned
Kale was eaten only when there was danger of starvation ( inreality they weren't going to starve )
Keep a check on their pee and pooh .
Pee is cloudy usually , and plentiful
Pooh losenges are non smelly , non slimy (unless you see a first passing - the ones they skulk off to eat) and either scattered about as they walk'n'pooh or in a neat little pile where they sleep .
Size shape colour should be consistant . (Though one of mine did poohs the size of a Ferrero Rocher once a week or so, normal the rest of the time . One did triangular flat based pooh ...no idea why
)
Loose pooh /liquid pooh is bad in piggies . If it doesn't resolve quickly its a vet visit .