Adding the at least 0 to the first level simply creates consistancy in the triggers; that way, if he compares them to look for errors if the conditions are more complex or whatever, it won't throw you off. I do that sometimes for more complicated stuff

If you upgrade by getting a certain amount of kills, do atleast and at most. And remember, at most and at least are INCLUSIVE, meaning you want the "at most" to always be 1 UNDER what you upgrade at. For example, at most 99 kills gives you level 1, and atleast 100 kills gives you level 2.
If you upgrade by other methods, just have a switch set or something else showing what level they are on like gas or score or something.
You could always do REMOVE all ghost if you don't like KILL all ghost but still want to do it that way

Also, be careful balancing that. Mass ghosts can really pwn mass zlings sometimes, lol.