[mac5732]

I like the ideas for the 3 types of monsters mentioned, this way everyone has their type of monster incorporated into the game. This could possibly be set as a random factor, that way you would never know which of the 3 you were going to see in your games. You could have 1,2 or all 3.
I would also make it an option that way those who didn't want monsters would be able to turn them off.

In some of the other sci-fi games out there, monsters were incorporated into the games and worked fairly well, ie; Moo, Federation, etc.

just some ideas mac