If a customer wants to dispute a PIN point of sale purchase that was done on his Visa Debit card, would I need to follow Reg E rules or Visa guidelines? And does it matter if it's one of the seven errors recognized by Regulation E?

It's very likely that, although the card carries the Visa logo, the transaction was routed through some other network such as NYCE or another of the regionals that offer PIN POS. If that is the case, Visa rules won't apply at all, and all you're left with is Regulation E. And that would mean that you'd only be required to address an error claim as defined in section 1005.11(a)(1), which lists the seven error types.