I think there is not a strict rule; it depends on the vegs you find at supermarket and the personal tastes of your piggies. It is a pity that they don't like peppers, but it does not matter as you can replace them with different foods. Personally I follow the rules suggested in Guinea Lynx website (and its calculator for dosing calcium/phosphorus), but I do that only for preparing the dinner. My piggies are fed mostly with different variety of grass (I have found also the charts with their amount of minerals/vitamins).
At dinner, my piggies can eat grass as they do during the day or I can dose the only (few) vegs they learnt to eat at the rescue: lettuce, endive, radicchio, pepper, cucumber (they eat only its peel), wild chicory, leaves of celery, and nothing else... they don't like carrots (only the green, but it is too rich of oxalates), kale and other things.
I try to avoid vegs rich of oxalates...
About fruits I offer them only as a treat, a very little piece of apple to Calipso and, if it is available, a very little piece of melon to Calliope. But these are only little treats once in a while.
Their main and favourite food is grass and hay.
As you can see, rules can be different... The only good rule to be followed is respecting the balance calcium/phosphorus. Read more on Guinea Lynx (as my vet commanded me to do).
