Wood and paper based litters are fine if they are not scented. Read the label and they should say pine oil/scent or cedar oil if they are scented. Don't use them if they do. Clay, sand, corncob, or any other litters made of small pieces or that are dusty are dangerous.
Here is another link to bedding materials :Guinea Lynx :: Bedding . It has basically the same info as the guinea cages link , but with a pros/cons section. I would cross reference since the guinea cages link has photos of the products.