Is it one fic with elements of Alien AND Predator? In that case, it's a crossover, and it goes in Crossovers.

If it's something actually based on Alien vs. Predator, I would guess that gets its own category in either Films or Video Games, depending on which iteration the fic is based on.

If you've got two fics, one in Alien and one in Predator, each fic goes in its own category in Films (probably). If the categories for the specific movies aren't there at the moment, just add them--but make sure they're in alphabetical order.

On a completely unrelated note, why the random line breaks? The text box you type into will wrap your lines automatically, and it doesn't reflect the width of the page once you post anyway, so it's not like there's no room for a long, continuous line. O.o

~Neshomeh