how would we define companies that provide payment services but are not typical FI's, for example like payroll services? (ie ADP, Paychex, etc) are these considered FI's? who would be their main regulator, would FED have any jurisdiction?

thanks in advance.

If they are publically traded then you could say the SEC.

In many instances, it's the IRS. You can select the IRS as the primary regulator when registering to be a FinCEN electronic filer. I believe that's reserved for those instances where a company may have BSA/AML requirements but is not a financial institution.