Welcome to the forum!
1. No, that food isn’t the best for guinea pigs as from what I can see it is a muesli type nugget, which leads to selection of the tastiest pieces and not proper nutrition. What you are looking for is a plain pellet, I can’t actually find the nutritional value of the stuff you have there, perhaps you could also post a picture of the ingredients and someone with more knowledge than me may be able to advise you more on the nugget you have.
A type that is favoured by a lot of members is science selective nuggets -
https://www.petsathome.com/shop/en/pets/science-selective-guinea-pig-food-4kg - but I am not sure how widely available they are.
Guinea pigs should have only 1tbsp of nuggets per day, so if you have two piggies the most nuggets they would get would be 2tbsp, but most prefer to not feed that many as they only make up a very, very small percentage of the diet.
2. The long thin line of poop/clusters can indicate dehydration or a stomach upset of some sort. Do you have water bottles or bowls for your piggies, and if so have you seen them using them at all? Are you sure they work?
All About Drinking And Bottles
3. As for veggies, piggies can have say a “slice” of bell pepper per day. I will attach the forums guide on this as it has a picture of an ideal ‘veg per day’ laid out on a plate that you may find helpful to adjust to your piggies day (post #2). Hay makes up the majority of the diet so ensure they have access to a large amount 24/7.
Long Term Balanced General And Special Needs Guinea Pig Diets
Most owners don’t stick to exactly what the image says, I know atleast my pigs diet gets changed based on the season/price, but it’s a great place to start. I only buy romaine lettuce, pepper and cucumber for them. I have herbs growing outside and all extras just depend on the day.
Is it bell peppers that you are feeding? Is that the only veg they get daily?
Additional to this, I will attach the forums very handy new owners guide that contains the links to answer any question you may have.
New Owners' Essential Information and Practical Tips Starter Collection
Feel free to share pictures of your piggies if you would like to, we love a good pig picture.