ok so if we look at the star wars/ halo wiki, at the hight of the empire their fleet was more than a million ships, and that would include the death star, eclipse ( with a smaller version of the death stars weapon) and the executor. the covenant had only a few thousand ships at the most, when it comes to the force. we have seen sith able to make suns explode with the force ( naga sadow ), the precursors and forerunners would have been dangerous, that is if the flood had not been there to destroy them. after all the rebels would wait for the empire to finish fighting before striking them at their weakest, the flood wouldnt give a -blam!-.