I wouldn't use pee pads as guinea pigs will chew on them and the materials can be harmful to them. What kind of substrate are you using in the cage that you don't want them just using that as their bathroom?
The plastic disposable wee pads used for puppies are not a good idea since they could be chewed and swallowed. However, wee pads made with fleece and an absorbent inner layer work well in the corners off the cage to keep the fleece floor clean longer. I set the hideys on them and my guinea pig seems to like the extra cushion.