  I think the best way to think of it is that the USSR essentially represented a third side which made alliances with whomever was convenient.  Early on, the USSR was considered by the Allies to be in the Axis camp if not officially, then by action; during the Winter War, there was consideration by France and the UK to send troops to Finland to aid against Soviet aggression (though they also planned on leaving troops stationed in Norway to hinder German access to Swedish iron).  It was only after being attacked by Germany that the USSR joined the Allies, and even then it was out of necessity, basically an enemy-of-my-enemy thing.  As soon as the German's (and Japanese) were defeated, they went right back to being their own again, hence the Cold War.  So, no, I don't think putting them as co-belligerent is to much.  In the grander scale of mid-20th century history, they were basically just a co-belligerent of the Allies as well.  