IMO a communist Germany is really unlikely, unless there is a radically different German Revolution, after that a surviving Weimar Republic is much more likely than a communist Germany (even in the absence of Stalinist doctrines of social fascism, KPD and SPD mostly hated each other, the Weimar SPD had no love for revolution, and the KPD, or even KPD and SPD combined, were never strong enough to take power via elections or by force of arms).
Either way, fascist Italy and fascist Russia are far enough away from each other to be friendly and both are revisionist powers interested in overturning the international order established at Versailles and enforced by the Western powers (Mussolini wants his new Roman Empire, Russia wants Poland, Finland, the Baltic States, Bessarabia back) so it is reasonably likely that an agreement in regards to the Balkans could be found: Mussolini gets a free hand for some border revisions with Yugoslavia, Austria, Hungary and maybe Czechoslovakia in his sphere of influence, a free Hand in Greece and support for expansion in Africa, Russia gets a free Hand in Eastern Europe and the rest of the Balkans (provided Italy can buy the needed resources at a good price), an agreement about partitioning Turkey might also be in the cards if the circumstances are right (not saying the agreement will look exactly like this, but I think there is a possibility of an agreement between fascist Italy and fascist Russia).
IMO Mussolini is ideologically flexible enough to downplay his Slavophobia if it gains him a powerful ally (after all he was against National Socialism, before he allied with Hitler), and Russia has enough on its plate to be bothered by Mussolini’s attitudes and might feel it is worth sacrificing some Yugoslavian border territories if gives them an ally that can help them (either against a communist Germany or by keeping France and Great Britain on their toes and keeping them away from Eastern Europe).
The diplomatic situation in Europe depends on what exactly happens in Germany and what choices France makes:
1) Communist Germany in 1919 that somehow survives Entente intervention: France is terrified of Germany and dislikes communists, so an accommodation with communist Germany is out of the cards, so sooner or later it has a choice to make in regards to Eastern Europe: either try to renew the Russo-French alliance (possibly including Italy) and throwing the Eastern European states under the bus or try to prop up these Eastern European states against both communist Germany and a fascist, revanchist Russia (basically trying to fulfil Pilsudski’s Intermarium project). Of course, France will try to balance things and might even be successful in that regard in the 1920s when fascist Russia is still weak, but in the 1930s or the 1940s at the latest this will be no longer an option.
If France choses the Russian alliance then you could see a weird alliance of convenience between communist Germany and countries like Poland (the German communists are still nationalists so Poland would have to make some concessions in regard to Danzig at the very least), the Baltic states and possibly Romania that are in danger of being swallowed up by Russia or made a puppet state. Great Britain might offer some support to this alliance against France/Russia/Italy out of a concern for the balance of power, especially if Russia is making aggressive moves in Asia.
If France choses Intermarium then you have basically a three-way-cold war going on in Europe with communist Germany trying to export the revolution while not provoking its neighbors enough to declare war on it as communist Germany would be too weak to stand against France and its Eastern European allies and even if it won, there would still be fascist Russia to contend with. Should fascist Russia invade Poland or the Baltic states and trigger a European war, communist Germany would likely support France and the Intermarium against Russia while trying to improve its position at the same time.
A reverse Hitler-Stalin pact is unlikely as an autocratic Russia is the historical bogeyman of the German left and it is unlikely that both communist Germany and fascist Russia have undisputed leaders that are as cynically ideologically as Hitler and Stalin were.
or
2) Weimar Germany survives: with Austria-Hungary removed from the picture there is little conflict of interest between Russia and Germany and so a Russo-German agreement to carve up Poland becomes very likely (Weimar Germany entertained such plans even when the prospective partner was the Soviet Union) and Germany has no interest in the Balkan as long as the resource needs of the German industry are met and Germany can hold a dominant position in Central Europe (which Russia would likely be willing to concede to Germany).
Now, both Italy and France have choices to make. Mussolini can decide to sign on with a revisionist Russo-German alliance (Weimar Germany is unlikely to be willing to risk a war with the Entente but would try to push the envelope) as a junior partner in the hopes that he can gain some things (free hand Greece, maybe some border revision with Yugoslavia, free hand against Ethiopia) or he can sign up with Britain and France in the hopes that he can gain some things (free hand in Ethiopia, territory from Yugoslavia if the country is allied to Russia).
France has to decide whether double down on its Eastern European allies (Intermarium again), and hope that they and France/Britain can stand against Russia and Germany together or try to pressure Poland to make some concessions and entice Germany into being friendly with France and remaining distant to Russia (or at least not too friendly with Russia, apart from economic relations).
Of course, either way France will do its utmost to avoid war with Russia but it might have its hands forced if Russia invades Poland; if Russia or Germany are allied it can either fight now or play second fiddle in a Russo-German dominated Europe forever, if Germany is neutral to pro-France not fighting would destroy France’s diplomatic credibility in Europe; appeasement of Russia is a possibility in regards to the Baltic States but throwing Poland to the Russian wolves would collapse the whole Intermarium alliance.
A Russo-French alliance is much less likely as it would very likely lead to very close Anglo-German relations as a response (Germany would feel threatened and look for a possible counterweight, Britain was friendly to Weimar Germany and would feel uncomfortable with a fascist, revanchist Russia), France would have to throw its Eastern European allies mostly under the bus, plus Weimar Germany isn’t that scary to France, especially in the later 20s, and Russia has little reason to be hostile to Germany, which means France would have to offer a lot and that makes it unlikely.
