While I'll not even attempt to debate the "pivotalness" (that's bad I know

) of one character over another, I think Serpico, that CnC and Walter's point is mainly this:
We are in the story for Guts, his plight, his vengence, his outcome.
Everything Puck, Farnese, Serpico, Isidro, or Schierke do is to somehow compliment Guts, this being Miura, his supporting cast have more recognition, memorability, in short a life of their own, that does not comply and may even conflict with Guts goals.
Be that as it may, Guts remains central (truly pivotal) while the rest of the cast exists to elevate or aggrivate him.
That's why the question of wither or not Serpico can beat Guts is somewhat superfluous, even he does, we will be more concerned with how that duel effects Guts then Serpico.
As I said before, it's not that Serpico can't beat Guts, or even if he's a better fighter or not (I think their too diseperate to compare but I digress) it's just that Guts is the major player, and as such, Serpico's goal are secondary.