I think 100 men with trucks and guns could do it.
Next gen solar powered electric joints that can never run out of juice and the ability to make as many bullets as needed...
But the way to do it would be by shooting only when necessary. Like. Run up in the Senate, smoke any guards on the way in just to show what it look like, take all the leaders hostage and then hold the spot down in a siege.
As they attempt to regroup and counter attract, kill any troops that get close enough and just hold the square
Then start issuing demands. Spread out the 100 men to some sort of perimeter slowly. As big of a square as they can comfortably hold.
Basically let all the mechanisms of trade and such continue, just have the proceeds handed over to the Brehs.
Only enough to live on would be fine at first. Slowly amassing more. Feed the people with the extras as you accumulate extras. Gain loyalty of the locals.
Make them into troops that can go out and do our bidding
Maybe send a few Brehs with gats out to lead a raid here and there. But mostly keep the guns in the original central perimeter and focus fire on any troops that try to test.