I think the course of the battle would determined by the time of mobile suit. If we’re talking original series mobile suits, the Empire would stand a better chance than if they went up against the Gundam Wing or Seed suits. Wing Gundam’s Buster Rifle or Zero’s Twin Buster rifle would make short work of the bulk of the Empire’s forces. Add in Heavy Arms’ armaments that could provide long range fire support. Bring in Deathscythe with his cloak and he can get in close to the walkers while Sandrock and Nataku maintain a perimeter, keeping the Empire’s forces within a certain area.
Most of the G Gundam suits would require very close quarters combat (with a few exceptions), so I think a large Imperial presence could give the Gundams some trouble.
Seed’s suits, I think, would be on par with the Gundam Wing suits in terms of being able to hold their own against the Empire.