To maximize system security, implement any payroll system as a separate instance on a dedicated server. We use SAP HR/Payroll in this manner. We use the SAP integration to financial systems, though we formerly used a different HR/Payroll system and integrated it to SAP financial systems, too. So, both approaches will work. If you already have SAP financial systems, SAP support will be easier if you go with SAP integration instead of a 3rd party.