Any Store bought Pellet is ok FALSE
Most pellets available in stores contain ingredients that are not suitable for consumption by your pet cavies.
Any pellet that contains dried fruits, vegetables, nuts or seeds is not a quality food. The extra pieces are either too high in fat and / or sugars, and present a choking hazard.
As for the plain pellets, for an adult cavy (over 6 months) you want to find a pellet that is made with Timothy Hay as the primary ingredient. Read the label, some pellets claim they are Timothy Based, however it is the 2ND, 3rd or even further down the list ingredient after Alfalfa, wheat, or corn.
A young cavy (under 6 months) and pregnant or nursing mothers should be fed an Alfalfa based pellet.
A pellet should also not contain animal products, such as meat, fat, whey, egg, animal digest, or bone meal. Guinea Pigs are strict herbivores.
The pellets should also be Ethoxyquin free, Ethoxyquin is a carcinogenic, cheep preservative.
While this is not the most comprehensive list available, it should be enough to get you started. More information in how to spot a quality pellet can be found at Guinea Lynx:
http://www.guinealynx.com/pellets.html
As a general rule, any pellet that can be purchased in a grocery store or Walmart / Discount department store is not a quality product. Many people do not understand why poor pellets are available, however it must be kept in mind that pet food is often a clever way to dispose of non human grade product, and waste products from food processing plants.
Any "Plain Pellet" is OK FALSE
Just because the pellet you are feeding does not contain any fruits / nuts / seeds / or other coloured pieces, does not make it a quality pellet. See above.