Human nail clippers are fine, but I personally would invest in a pair of small pet nail clippers.
If you compare the 'blade' on human clippers vs pet clippers you will see it is a lot thicker and more blunt.
Pet clippers have a much finer blade, so have more of a direct cutting action.
Human clippers will cut guinea pig nails, but also squeeze them more because of the thickness of the blade.
This isn't an issue for humans as we don't have a nerve inside our nail like guinea pigs do.
Human clippers can cause some pain as the squeezing effect (due to thicker, duller blades) can compress the nerve slightly and cause your guinea pig to startle.
Pet clippers aren't expensive and are well with the investment.