Please be careful using human shampoo; they are pretty harsh on the skin, ESPECIALLY nizoral, which is pretty aggressive even for human skin (and yes, I have used it when I picked up ringworm from my piggies some years ago)! Even the mildest baby shampoo can contain substances that are not good for regular use.

There are now better and milder antifungal medicated dips and shampoos around, which should ONLY be used in case of diagnosed fungal and with the consent of the treating vet. We do no longer recommend to treat on spec with human products; the majority have turned out to be less than ideal to downright harmful.
If you want to use shampoos with a ph for guinea pig skin, then please look at Gorgeous Guineas products, which can be delivered worldwide. Generally, we no longer recommend bathing unless absolutely necessary. In most cases, a bum bath with hand warm water will do the trick.
Gorgeous Guineas - Aromatherapy skincare products for guinea pigs