Neem oil is safe on guinea pigs and if you search online you can find a lot of pages about its use. Also in "Peter Gurney guinea pigs health guide". He says (talking of parasites): "One of the best treatments for this is Neem oil, at a dilution rate of four parts of carrier oil to one part of Neem oil. In Asia this is used as a insecticide against the malaria fly. In case the word insecticide alarms readers let me hasten to add that it is also used to flavour food, hence it’s safety for use upon guinea pigs. To underscore the food flavouring side of things, there is a side effect, your guinea pig will smell like a Chinese takeaway dish when you massage the oil in!."
Try to make a simple research on Google, I see there are a lot of info about the use of these oils and don't expect a vet or a doctor is an expert of holistic and herbal treatments. This is another subject.
(the old pig I owned long ago totally recovered from an eye infection washing it with plantain boiled in water; plantain has strong antibacterical properties, but the vet showed interest in this remedy only after seeing my piggie's recovery - without any drug as he prescribed- ).
Have a good study online and let us know the results!