Hey didn't you notice that the Predator has a long Jaw. Well a facehugger jumped onto the Predator's face and the would be pushed back in and the facehugger cannot get through into his mouth to start breeding. It was a good idea though. I am not saying it is stupid. Having PredAlien all in one would be the best thing we ever seen.
It's just that it has a little sharp teeth on eacth corner if he opens his jaw. You need to look at the Predator'e face, and you will know what I am talking about.
maybe the facehugger is flexible, who knows. The facehuggers have facehugged stranger creatures before on different planets with the indigenous life forms to survive im sure they could handle a little predator.