Hello,
I got a lot of experience with leveling on Centaurs, Floaters, Aliens and anything you can find the the desert. I noticed the one thing. As we know, each desert-living creature have some sort of "special" effects of it's attack like Aliens - radiation, Centaurs - radiation and poison, Floater - poison
The thing I think it's a bit unfair is the experience you got for killing each creature. The Centaurs have lower HPs than Floater, hovewer their attack is more dangerous than Floater's. So my suggestion raise the experience points you get for killing Centaur to 750 and 1000 (for normal and mean one) and lower the experience you get for killing Floater to 650 and 900 (for normal and nasty one) since Floater is less dangerously to fight with.