https://www.opm.gov/policy-data-oversight/pay-leave/pay-admi...
Doesn't sound like you're talking about General Schedule.
Correct and there’s no legal requirement to use the GS.
This new force could easily and legally acquire and pay through other schedules - happens all the time.
I was hired in under HQE accession in 2019 and made SES 4 equivalent with zero civilian time in service.