I personally almost never use the stealth kill button, but when I do I try to make sure they are alone, but with a map so small any alien can run across the map during your process, and reach you before your finnished.
For that reason I hardley ever use it, instead of a trophy kill I'd rather throw a spear at his face while he's on the ground or something to that matter.
I cant imagine stealth killing ANYONE on an 18 player game without dying just after it, unless of course these maps are huge which I highly doubt given that there will only be 8 of them. If anyone ends up stealth killing someone and surviving with that amount of players with maps the size of the demo or slightly larger, it'll be because of shere luck, and nothing more.
Of course I could just be talking out my ass right now as I've not experienced an 18 player game in this, so yea....