I think this question is rather academic because the way you've described the scenario the implication is that it's actually irrelevant whether the neutrals join the Allies or not...So, I'm working on a bit of alternate history.
To keep it quick, the German revolution and Biennio Rosso both produced socialist states, the Hungarian Soviet Republic survived with russo-german intervention, and Poland, Finland, and the Baltics all were brought into the fold either as SSRs or "independent" socialist states. Also, there's a Rhenish state under French control.
My question is, as alliances form, tensions rise, and a new sort of WWII seems increasingly unavoidable, what states that were neutral in our WWII would be aligning with the Allies here? Sweden, Norway, Denmark, Bulgaria and Turkey all have the reds right on their border, Belgium and the Netherlands are both almost certain they may become battlefields, and have no sympathy for Germany besides, Yugoslavia is subject to Hungarian territorial ambitions, and Greece and Albania are surely wary of the italians' aims in the Adriatic.
I'm not sure just how many would really throw in with the Allies for a war, though.
...because the giant Comintern you've created will invade them anyway to "bring the revolution to the capital-fascist regimes", thus dragging them to the side of the Allies.
Likewise, it is certain that the Red Scare will be even greater and I would not be surprised at all if the United States and/or the United Kingdom become openly fascist, seeing that basically all of Europe east of France has turned red.