This is an easy one for those of you already in the bidnus, but as an outsider it's not obvious. Browsing the pay scales here on the forum for the different airlines, under Captain, are the number of years shown on the scale since becoming Captain or total with the company? I would think the former but just want to make sure.

At any reputable airline it will be total years with the company, not just years as captain.

Why even have a one then? Does anyone ever start out as a captain with the majors here in the US?

Yes, some regionals have been known to hire Direct Entry Captains and some majors currently have upgrades at the 1-2 year mark. It's not inconceivable that it could go lower and someone could hold the seniority to be a Captain within their first year.

It would be very unusual at a major. But, it costs nothing to publish a pay scale for each seat starting at 1. If not, where do you start? Is two unnecessary? three? Impossible to know, so why not just start at one?

Usually the total seniority jumps over to the left seat pay.
Have only heard of 1 company where a 5 year FO started over again at a 1 year Captain payscale. F that.