A little tidbit on monsters - one that applies, in general, to single player, since 99% of people play with monsters OFF in netplay.
Make sure you know which monsters apply to your levels. Does it really make sense to put Enforcers on levels with lots of water, like "Thing What Kicks"? Does it make sense to put them on levels where there's not much room to maneuver, like "One Thousand Thousand Slimy Things"? Does it make sense to put them in a heated battleground, like "What About Bob?" The answers, respectively, are no, no, and no. Consider not only the plot purposes of a monster when you apply it, but also the environment that makes them threatening without being unfair (remember, you don't want to annoy and frustrate the player, just challenge him).
Also, put a theme of monsters on each of your levels. By that I mean, you shouldn't put every monster on every level. On this level, you should put lots of Troopers and Fighters. On that level, there should be more S'pht and Cyborgs. On the other level, put in loads of Hunters. On the next level, combine Hunters and Troopers. It's not so much an issue of good application of monsters, here, so much as a creative use that gives each level a more unique feel (example: "Aye Mak Sicur"). This isn't to say that you can only use one monster on your level - spruce it up, but keep it unbalanced one way or another - if anything else, it keeps the player from being confused.
Lastly, think about it from a plot perspective. The Pfhor wouldn't deploy their "Random Amalgam of Guys Group 5," they'd deploy their "Hunter Task-Force Group 5," or their "Fifth Trooper Invasion Divison." Also, it doesn't make sense to place Flick'ta right next to Pfhor if you don't want them to be fighting when you get there (remember, they're enemies by default).
