Hay needs to form 75% of their daily food intake.
Any grass hay is absolutely fine.
It doesn’t need to be branded hay, it can be meadow hay from a farm for example. It just needs to be fresh and sweet smelling.
The four veggies they can be given daily which provide a good mix of nutrients for balanced nutrition are cilantro, lettuce, cucumber and bell pepper. Try to include these four every day.
Fruit, carrots should not be in the diet regularly - they contain too much sugar and too much sugar can causd an overgrowth of the wrong type of gut bacteria. However they can be given one very small piece as an occasional al treat. No more than one piece once a week.
Spinach, parsley, kale are all very high in calcium and should be given no more than once a week. Too much calcium can lead to the formation of bladder sludge and stones.
All other veg should be rotated for variation and balance and given in moderation.
Pellets are the least important part of the diet. They don’t actually need any at all if they get a good balance of veg.
Whatever pellet you feed, ensure you give just one tablespoon per pig per day.
Less can be better! Mine only get pellets 2-3 times a week but they get plenty of fresh grass and forage (fresh in summer, dried in winter) instead
Long Term Balanced General And Special Needs Guinea Pig Diets