let's say my piggies (and I) are a bit WEIRD...
(and proud to be like that)
Anyway, this is my own experience with my latest two piggies: I adopted them when they were 3 months old and since the day of their rescue (when they were 2 months old and not in a great shape) they were educated to the correct diet, following the prescription of the vet: hay, 1 tea-spoon (or less) of grainfree pellets, 100g of vegs (always a piece of pepper) and a lot of fresh mixed grass. One of them was underweight, but she started eating a lot and her weight has doubled now. Their tummies never had problems. They DON'T drink a single drop of water... I was astonished and shocked about that, therefore I had a talk with that first vet and with the lady who looked after them at the shelter. The vet told me it does not matter at all (their wee is copious) and I can offer vegs and grass a bit wet. Actually the piggies don't like eating wet grass, I see...
Now they are checked by another piggie savvy vet, one of the best in Italy... and he also told me to feed them with no more than 1 tea spoon of dry food and a lot of wild grass (I live in a district with a lot of green areas), at least until we have some... because just now most of grass has spikes and in summer the areas will look like desert. I pay close attention of the ratio Calcium : Phosphorus, you can find a calculator in Guinea Lynx website, but the common grass (the one with strings) is maybe the only veg having the best balance and a great quantity of vitamins and minerals.
I have read somewhere that grass has also a component which helps teeth to be "eroded". I cannot believe that a strict herbivore as guinea pig is can have some trouble eating natural grass...
This is the great amount of fresh grass my piggies ate today for lunch:
only a thing: they are becoming two great gourmets: they prefer a certain grass and a certain wild chicory... I have learnt where the right grass grows up... but sometimes they simply don't eat because the grass is not the one they prefer... and they refuse to eat also lettuce then! when it is grass time it means that grass must be served. No matter that outside it is raining...
If your piggies will get used to the grass, you will realise the grass is a rich food, something like a steak for us; they won't be too much hungry then.