Have you play tested with 5? Maybe you could make some official recommendations as to the advantages to give.
I play games with a full five player count quite often and would love to be able to do so with this game.
We've playtested the 4p coop, and *I* needed at least 2 "advantages" at setup to win. For comparison, I can usually win a solo with up to 2 "disadvantages", so I consider myself a fairly strong player
After that we said that making it against a live opponent (who will always target the weakest link) is nigh impossible. If you want to try it out (you can figure out the rules based on how 4p coop is handled), I'm not gonna stop you, but we give no promises that it'll end in anything but Soviet victory due to injuries or morale on turn 2-4.