Our recommendation regarding which employees should be included in your documentation is that all monthly employees should be included, not necessarily hourly employees. The reason for this is that hourly employees rarely have an individually set hourly wage, and it is also not regularly renegotiated.
However, it would be very beneficial if you take the opportunity to review your hourly employees during the pay equity audit to ensure that the hourly wages are gender-neutral for the different positions in which the employees work.