Good idea using the playset figures. Works for something like this because the scale of the figures dont have to match that of the alien.
I saw another diorama that used some modified cowboy playset figures fighting a dinosaur. Looked really nice.
I wish I could remember the name of the book where the author showed how to modify playset figures for use in dioramas. It involved cutting off heads, arms, legs, weapons, etdc and switching or repositioning them.