In my limited experience I've never seen a guinea pig with fleas, although I don't doubt that they could get them since fleas can be pretty adaptable! A parasite you're more likely to come across with guinea pigs, and other small furries, are mites. I'm not aware of any spot-on type treatments for small furries for fleas or mites (firstly the dosing would be pretty minute and secondly small furries are often kept together and could lick it off one another), but you can use a pet shampoo on infected animals. You can be prescribed specific shampoos by a vet if the infestation is serious but many over the counter shampoos will probably work - I had a rabbit several years ago that contracted mites from his bedding, and after a couple of baths (and a new lot of bedding) he was fine. Hopefully Lucky will live up his name and not pick up either - and enjoys his time outside!