I haven't seen Prometheus so I'm not sure how powerful the engineers are, but there's no indication that the Yautja are a Galactic power, which is what it would take to fight the Republic, the Empire, or even lesser powers like the Pre-Confederacy Trade Federation or the Imperial Remnant.
As for the xenomorphs, they're also not a galactic menace. They take over ships, stations, and maybe planets if they're poorly defended, but even if the preds transport them on their ships and they don't fight each other they'll only be able to take out a few planets before the SW fleets get wise to them and hunt the pred ships down.