As for choosing monsters, I'll go with the consensus of adapting them to the environment though I usually don't bother too much with ecologies. I'm not running National Geographic Simulator nor am I playing Sesame Street:The RPG. Monsters were meant to be killed. (Sorry Elmo).
One can do with the idea from the Dragon Age video games which has various races tainted with the Darkspawn Blight. So Shrieks came from elves, genlocks from dwarves or goblins, hurlocks from orcs or humans, Ogres from the giant races etc...