I know this is kind of a controversial subject, but I believe pellets are unnecessary for guinea pigs. They are like multivitamins, you don't need to take them. Pellets have often also been associated with bloat (when fed alongside veggies such as cabbage, broccoli, romaine lettuce, etc.) and bladder stones. They also swell up in a pig's stomach, because of the liquid, you can experiment this, and thus lead to a decline in hay consumption, which is the most important part of a guinea pig's diet. My guinea pig, Flynn, gets all of his nutrients through vegetables and hay. This is actually how they would be getting their nutrients in the wild; I try to make it as natural as possible for my guinea pigs. Most pellets also contain grains, which I personally believe should not be fed to guinea pigs. There is a special pellet that is grain-free, but I can't get it because it is sold in the UK. The only time I would feed pellets is when my guinea pig(s) are sick (Critical Care, for example).
Feel free to disagree with me, this is just my opinion and I am not trying to make everyone do it "my way". A lot of people actually believe in this "no pellet" thing as well, but I understand that not everyone will agree.