In the picture, I'd imagine the facehumper would kill the headcrab, since it's on top and all.
But I wonder what would happen if you had multiple forms of zombie-creatures.
What if a traditional zombie invasion (Dawn of the Dead) met a species like in 28 Days Later (Rage Virus), or met Headcrab zombies (Alien Parasite)?
Which zombie race would be top dog, or would none of them fight each other because they would all seem like zombies to each other?