Nizoral shampoo is very rough on guinea pig skin, as is any human anti-dandruff (i.e. anti-fungal) shampoo. It can be used as a one-off end of treatment shampoo and for treating yourself if you actually catch ringworm from an acutely affected guinea pig.
Ringworm is typically transmitted between neglected or highly stressed commercially mass bred pet shop babies but unless you are actually dealing with an acute outbreak, there is no need to treat for ringworm.
We have a very detailed practical ringworm guide which tells you how you can get on top of ringworm once and for all without any need of 3 monthly bathing - when you are actually confronted by an acute outbreak. If you follow it closely, it does really work as forum members can affirm. Our collective practical experience on this forum and a systematic access of eliminating ringworm spores from every single possible transmission angle have gone into it. It's now had over 65,000 hits.
Ringworm: Hygiene, Care And Pictures
With around 90 mostly adopted piggies and several decades of ownership I have only ever had to deal with three outbreaks, carried in by a new guinea pig in last 20 years. No preventative baths needed.
The last outbreak in 2017 I could confine to just the one affected guinea pig who had been previously surrendered to a vet for euthanasia due to prolonged inappropriate DIY treatment for very advanced ringworm. Nevertheless, with the correct treatment he made a full recovery and - after following my own advice carefully - never suffered from ringworm again for the following 5 years of his life; he died a year ago. Nor did any other piggies in the whole piggy room.
Please be aware that conditions in the USA are very different (especially in terms of vet access and a high percentage of sold guinea pigs being diseased). Recommendations cannot always be transferred directly to another country, especially when it comes to brands and home treatment. There are comparable European veterinarian products available in your country.