The standard Republic, Special Tactics and Naval troopers are available to build in Era 1 and certain historicals, which is a different lock method to the Heavy Republic Trooper.

To make those three buildable, you need to navigate to Data\Scripts\Library\eawx-states\tech - from there, you need to open tech-era-two through five, then remove or comment out the mentions of the Republic Troopers, Special Tactics Troopers and Naval Troopers.

You can use the same method to unlock units early (HMP), keep units from being locked (AT-XT), make certain units buildable (PDF DHC) or lock other units entirely.

Ahh, see, I had assumed there was a file for all the tech-by-decision advancements and such but, thinking it would be amongst the XML files, i wasn't able to locate it. Thank u