That sounds like we need a hard-coded setting complementary to the "Ground Combat Militia Troop Population Amount" settings. This would be the
Troop Occupation Level -- the number of troops necessary to control a population. It's a perfectly realistic idea, as anyone paying attention to the Iraq war would already know.
If you don't have enough troops to keep up a sufficient presence, the occupied population will revolt. Maybe the original empire doesn't control the planet anymore, but neither do you.
Alternatively, MM could just use the militia setting
as the occupation level. Which also makes some sense.