Guinea pigs are grazing animals, optimally their diet contains nothing but grass. The reason to feed pellets is to provide vitamin C and certain elements.

The good kind of pellets are homogenous, they don't contain any mueslis or colourful bits. Although the number of pellets made for cavies are almost unlimited, only a very few of them are suitable.
When you are looking for the right kind, make sure that the crude fiber content is at least 20%, while the crude protein in it is no more than 14-15%. The ingredients cannot contain grains, and the calcium-phosphorus ratio must be right (approximately 1:1.5).
Grains contain too much starch, and starch alters their microbiom in a wrong way. It reduces the efficiency of their cellulose digestion, vitamin production and the bacterial protein synthesis.

The most accessible suitable pellets are Oxbow Essentials Adult Guinea Pig, Bunny Nature Guinea Pig Dream Basic and Versele-Laga Cavia Complete, the latter two can be ordered via zooplus.
These pellets cost more than the ones with poor ingredients, mueslis and seeds, but one guinea pig needs only two spoons per day. Do not overfeed them. The more pellets they eat, the less hay they consume, which can change their microbiom, ruin their teeth and make them obese.

Young, weak, sick, pregnant or nursing cavies need more calcium and/or protein. In their case we can use alfalfa-based pellets (for eg. Oxbow Essentials Young Guine Pig) or supplements like Oxbow Critical Care - the latter should be part of the guinea pig medical cabinet.

Guinea pigs are not seed-eaters. Seeds can cause physical harm, abscess and obesity.  Do not ever feed seeds. However, you don't have to worry about the seeds that might be present when feeding grass or hay. Those seeds are soft and tiny, and their amount is negligible.

When a guinea pig consumes unlimited grass, you don't have to worry about the vitamin C. 100 g of fresh grass contains approximately 50 mg of ascorbic acid (vitamin C). A healthy adult needs about 10 mg, pregnant or sick animals needs 20-30 mg.
High quality pellets contain stabilized vitamin C, so that can provide the necessary amount even if the animal live on hay.
However, vitamin C tends to decompose quite quickly. Once the package is opened, the amount is getting less and less. So, to make sure to avoid scurvy, when the only source are their pellets, you should provide supplementary vitamin C about twice a week.

You can supply vitamin C as a drop, or simply as ascorbic acid powder. In both case it is important that the daily dosage must be consumed immediately. The easiest way is to place them on a miniscule amount of veggie, that the animal eats quickly.