Just as we will strive to flesh out the characters, places, and background of the Rebel base, we will also work to detail the main opposition that the RPG PCs (or X-Wing players) will be up against.
In this case, the Sumitra Sector Imperial forces. Since Sumitra is a relatively obscure location, the overall Imperial leader (in direct control of the Imperial Navy and Army units in the sector, as well as the boss of the civilian Imperial governors of the various planets) holds the rank of Moff (not 'Grand Moff', a title reserved for High Priority Sectors).
The Sumitra Navy fleet is likewise smaller and less imposing than can be found in other areas of space. Likely the best vessel in the sector fleet is an aging Victory-class star destroyer. Not a top-of-the-line type capital ship, by any means, but still much more powerful than anything the Rebels have assigned to Sumitra. Numerous support vessels aid the Victory-class ship in carrying out the edicts of the Emperor. We'll look to add details to the various units assigned to suppress the Rebellion locally: TIE squadrons, stormtrooper legions, army units.
The war against the oppressive Galactic Empire is actually hundreds (or thousands) of individual, localized fights, each spanning a sector or so. One of the main goals of the Rebel Alliance is to keep up the pressure, militarily, across as much space as possible. Even low level nuisance activities aimed at Imperial installations helps to keep the Empire's fighting forces spread out. The Alliance is not interested in set-piece land battles, pitting army versus army, nor do they desire direct fleet-to-fleet confrontation. This is a 'death by a thousand cuts' type conflict, at least at this stage.