It’s not so much that you can’t feed things, it’s about moderation and finding a balance which is right for your piggies
The four safe daily veg are coriander, lettuce, bell pepper and cucumber.
Anything else should be fed in rotation and moderation.
Dark leafy veg, such as kale, spinach, parsley, are high in calcium and should be fed no more than a small amount and no more than once a week.
Actually more calcium comes into the diet via pellets and water than veg. Making sure pellets are kept to no more than one tablespoon per pig per day and that water is filtered will help.
However, if you go too low in calcium intake then it can also cause the same issues as too much calcium.
The two guides below will help you
Long Term Balanced General And Special Needs Guinea Pig Diets
Wiebke's Guide to Pees and Stones
My piggies get the four safe daily veg, lots of fresh grass, lots of hay. I hardly feed pellets - they get just 5 pellets each and even then they only get them twice a week. Water is filtered.
I do feed the high calcium veg on occasion but I have more leeway because I don’t really feed pellets.