UPMs can not only stand in for Mechs, they can supplement aerofighters quite well. The Scorpions should know, since they lost a whole WarShip to Aerial UPMs and aerofighters at Huntress. That's first-hand experience.
A touman of UPMs will enhance the durability of the Imperio in case of a full-scale Homie invasion. The little buggers have less resource needs and more importantly are much smaller than Mechs, making them easier to conduct guerrilla warfare and fade back into the wilderness. They can also go to/fight in places no vehicle can, and are to a battlesuit what a battlesuit is to an infantryman.
The Imperio created a garrison sub-caste for the natives to fight in. If they don't trust the natives fully with high-end tech, they can organize the natives in clusters of conventional units(clantech infantry,tanks and conventional fighters) that defend local areas and act as terrain guides. While the warrior caste/Touman have the Mechs, battlesuits, ASFs, UPMs, DropShips and JumpShips and act as the rapid-reaction force for plugging holes.
This organizational model will be useful until the different peoples are truly bonded and integrated. Then we can have units that are only loyal to the state rather than to an ethnicity.