There are multiple ways to set up your overtime rules in Harmony's Time & Attendance module. All rules are 'grouped' ("Overtime Group") and rules within the Group are sorted in order of priority (i.e. rules with a higher priority will apply first).
Standard import & export interfaces are already in the system by default. If you are not looking to import or export anything specific from Harmony T&A, you do not need to set up custom interfaces.
Your timesheet periods refer to the period of which your timesheets are for. Often, this is identical to your pay periods in payroll. You can review your timesheet periods by navigating to Setup > Timesheets.