Well one of the problems is the whole timeline. ME and Halo are two different Verse all together. Unless you are going to have the Forerunners be Prometheans but even then, Flood and Reapers are opposites of each other. See where I am going here, it would be a cluster just to figure out how things would work. Even though in Infinity Rises, it was more plausible with the Infinity being a Generational ship that reached the fringes of that other galaxy.