Player sprite question.

Do you guys know about the Doom64 (Skulltag) guy and Zandroguy skins?
I was wondering if you can use those as a base for modification to create new skins to use in a project.

Only if you use them in a product which you wish to make profit out of.

That's half-serious advice. You should contact whoever the authors, if they even reply back. If you still need it, give proper credits.