1) The predators arn't cowards. The whole reason they go down to earth to battle the Aliens is as a right of passage into manhood. In the books there are 9 of them that go down (Goes along with the storyline of the first 2 predator movies, 1 dies in the first, 1 dies in the second, and at the end of that second it shows 7) to battle Aliens as their right of passage. It's an ongoing war from since the begining of existance between the two species. The predators can be outnumbed 1000/1 and still win, because it's in their blood to fight until death, and if they are ever beaten, to destroy themselves rather than die by the hand of their enemy. Kinda like Samurai.