Not a lot of good options, I know. Sovs didn't have a lot of friends.
Yep, Stalin's endearing regime combined with Hitler-like expansionist rabid dog antics shall win you a lot of friends...
Given so few choices, I'd go with Chiang if only for mutual necessity. Don't forget, SU gave GMD quite a bit of aid in the SJW, & fought Japan: "the enemy of my enemy" thing.
Yep, but the point is, would Chiang even register as a worthwhile Soviet ally ? If the SU has to fight the British, the Germans, the Italians (pitiful as their contribution may be in comparison, but still), and the Japanese, not to mention the French when and if they put their house into order, all supported by an increasingly anti-Soviet USA with massive Land-Lease if nothing else, it has enough problems already fielding and supplying its own armies. Although I concede that in this scenario the SOviets would have little problems sending Chiandg supplies by land. I'm not that much convinced it could send supplies enough to make Chiang a worthwhile ally, albeit this scenario would convince the GMD and the CCP to make a really effective collaboration, which would improve things a bit for the Chinese. I'm also uncertain how many troops Stalin can spare to make an intervention in China and open a second front against the Japanese. Lack of troops with really decent training and equipment was what dragged Chiang down. Having plenty of that and some modern allied ground troops could change much.
Besides, in this scenario, the UK and the USA shall quickly bury any objection they might have had to Japanese expansion in China (afterwards it's a different issue), so the IJA would swim into oil supplied at generous prices by its own ally. Which should make them rather more able to fight both the Svoeits in Siberia and the United Front in China.
In sum, would China give any valid contribution to the SU, or be a burden like late Mussolini was to Germany IOTL ?
In the same vein, how does India grab you? I have trouble believing real hostility to Britain, but... Iraq &/or Saudi, which OTL did rebel against colonial rule?
Or, further afield (& increasingly less probable, IMO...), Brazil or Argentina?
I have given thought (and already included in the TL, even if I did not yet get to the point of giving it serious coverage, like France, even if it ought ot be next on the -very slow- line) to some serious Commie-supported insurgency in India (in other words, a serious speed bump for the Allies, but little more). I'm not convinced it could ever get any more success than the philo-Axis nationalists did IOTL. Afghanistan separates them from any serious Russian support. Could the Russians break out all the way to the Indus ?
As for nationalist Arabs, they lacked a serious pre-war Communist presence, and typically did not have much sympathy for Communism, but they joined that camp in the OTL Cold War, so yep it might be feasible, any devil to cast off British yoke. But again, would they avail to much ? If the encircled Soviets can make a strategic breakthrough in Caucasus, Persia, and Kurdistan (the Turks are no lightweights, but for the persians, it all depends how many British troops are stationed in Persia), yes, they can give support to the Arabs and alter the strategic balance as the Allies get critically dependent on DEI and US oil to keep fighting. Otherwise, they are yet another minor speed bump, and as the Allies row the Middle East machine-gunning uppity Arabs and in a few months all is over.
The Brazilians and the Argentinians are ASB in my opinion. Their various sponsors were the Germans, the British, and the Americans, all in the same side, and the ruling classes were fiercely anti-communist. The generals would eagerly put a gun to any president that dares think jumping in Stalin's boat, and machine-gun any mob that dares support it.
You should avoid it, IMO; as you predict, IMO, the Allies'd almost immediately invade to crush it.
Oh, yes, ultimately they would go down under Allied assault, but it would buy Stalin some serious breathing space (say half a year), which is more than you can say of any other alternative, except maybe an abundantly supplied Chiang. Also, it would rob the Allies from any serious contribution from France for almost an year, as they have to crush the pro-Soviet junta French Army, occupy the country, clean up the philo-Soviets, set up a new government, recruit a new French Army, reset French economy and in dustry to functionality... it's the worst "speed bump" for the Allies that I could think of.
Could you do it with a "Communist" win in the Spanish CW? (I can never keep straight if they're Republicans or Nationalists...

)
Outside the boundaries of the scenario, I'm afraid. I cannot see how butterflies from Hitler's death in late 1938 and British-German detente could save the Republicans. At this point they had already lost the SCW. Even admitting you compress the development of the scenario (which would skirt unplausibility) and have Stalin decide a massive last-ditch Soviet intervention to save the Republicans, this would just accelerate the unfolding of the scenario and the formation of the Alliance. Panicked British and French would hastily chalk Stalin as the worst meance in Europe, join Germany and Italy in grand anti-Communist alliance, Allied fleets would cut any supply to the Soviet expeditionary corps in Spain, and Allied expeditionary coprs would eat the logistically-starving Soviets in a few months.
Suits.

I can picture (fairly) big tank battles in the plains & use of airborne in the mountains: 1st SSF?
desantniki v Fallschirmjagern? Even (if it goes long enough) the
555h?
All of that, sure. I think this WWII should last long enough to see some American boots on the ground. Stalin and the Red Army started the whole thing (even if it exploded to a general war much before they had planned, mch like for Germany in OTL 1939), so they had plenty of warning to deploy their forces. There shall not be any massive Barbarossa Blitzkrieg, rather the Allies using their superior industrial potential, the superior quality of their forces, and theri encirclement position to slowly bleed the Soviet manpower reserves white, and cut Soviet industrial potential down, as they make deeper and deeper inroads into Russia. No one big Barbarossa Blitzkrieg IMO, rather a string of Allied mobile offensives rather like the reverse of Soviet offensives in OTL 1943-45.