Well, let's take a look at the general realms of conflicts, then:
Hungary: The biggest issue, because substantial Romanian occupation troops who had just been suppressing the HPR have just disappeared uptime, leaving bad blood to start. Transcarpathian Ruthenia is part of the chaos of the western Ukraine, and Transylvania is, well, filled with Hungarians. The current government in Hungary is also the most likely to fight of those in Europe, and the interlocking Slovakia-Czechia-Hungary-Serbia-Macedonia-Kosovo-Austria-Liechtenstein-Switzerland region is a massive network of self-sustaining economies that can trade with each other downtime... Except for oil, and guess what Romania and Galicia still have in 1919?
The Hungarians only have a light division active organized as two brigades, but they also have 160 MBTs and 550 BTR-80s including those in storage, and a substantial part of the Romanian regulars who were garrisoning Hungary just disappeared--which will cause fear, terror and national outrage in Romania. The Szekely will certainly rise, and nationalistic volunteers like youth supporting Jobbik will join the military in numbers. I think it's safe to say that Hungary and Romania are going to war.
Serbia: I agree with Psihokekec that in the circumstances Croatia and Slovenia will still quickly move to unification with Serbia and for that matter Bosnia will present no resistance, at least until Kosovoars arrive to start telling them what happened uptime. We should not ignore the strength of monarchist sentiments in the era however; I think there will be popular demand for a King for the new federation, but Montenegro conveniently has an ethnically Serb and Orthodox dynasty. The Serbians might send troops into North Kosovo to protect the Serbian villages, though; tensions will certainly be high. The Czechs are again probably the unifying diplomatic force trying to promote something like uptime European values.
Macedonia: I daresay, what would their nationalists do if Mustapha Kemal offered them an alliance against the Greeks, currently invading Asia Minor, and a chance to seize Thessalonica, which in 1919 is a substantially more Slavic city, especially the hinterlands? Macedonia may not be landlocked for long.
Belarus: This remains the most spectacular problem here. Like Psihokekec says, Lukachenko is quite likely to throw in with Lenin. The Belarusian Army has 4 mechanised brigades and 2 mobile brigades (plus a Spetsnaz brigade) in the active service, but it also has 120,000 semi-trained territorials and lots of old equipment to be reactivated.
Lukachenko, to his credit, is at least a little likely to be concerned about Great Russian Chauvinism. The easiest way to deal with this in the new Soviet Union is to give Belarus a very nice bargaining position. Expect Lukachenko to begin collaboration by moving into the Ukraine with his armed forces to complete the defeat of the Whites and Blacks and have the gripping hand in organising the Ukrainian People's Republic vis-a-vis Moscow (this is what guarantees I won't be born in this universe. It won't go well for my paternal grandfather pitting a 3" field gun against a T-72).
Mongolia - Now we reach the countries which would probably not do anything, except that Lukachenko committing to the Soviet Union will cause mass chaos. Mongolia is a functional multiparty democracy that is probably not at first going to think "ah, I need to go invade other countries". But once Lukachenko starts supporting the Red Army, this is going to change rapidly. The Mongolians have substantial quantities of equipment, and they need oil, they need equipment to run a modern economy--in short, they need access to the sea. They also need security against the Lukachenkoist Red Army ultimately coming east to beat down all comers who are not communist.
Quite frankly, in 1919 that security means some kind of arrangement with Japan. And in that context, Inner Mongolia is beckoning. I would expect the Mongolians to mobilise over winter and then move to occupy Inner Mongolia, which is in a state of total chaos, completing national unification, and then moving on into Manchuria. Expect diplomatic contacts with Japan, which is actually in its most liberal and democratic direction. The Japanese already have substantial troops in the region--We could easily see a more independent Manchurian puppet government rapidly formed in which uptime Mongolia and Taisho Democracy Japan are equal partners. Once oil starts flowing and rail connections are re-laid, the Mongolians can be confident they can hold off Lukachenko and the Red Army.
Kazakhstan - Actually arguably the most powerful nation here, since it can potentially seize the Cosmodrome and begin to orbit satellites for intelligence gathering purposes for as long as the store of rocket parts holds up. Kazakhstan will begin a mass mobilisation of troops--national conscription--to try and deal with the situation. Over the winter they will horde fuel for combat operations and will have to make a decision on whether or not to intervene. Expect them to ally with the Azerbaijani--who will be using their oil to advance their own political agendas--and Mongolia to form a triple alliance against the Reds and Lukachenko.
Kazakhstan's Soviet era leadership has faded in the background, and is much less communist-friendly like Lukachenko. I am fairly confident they will make it clear they have no interest in being made Red or collaborating with Lenin. But I also think there is still enough in the way of positive sympathies that they won't immediately move toward resisting the Soviet tide in Russia. They're in a much better place economically, though, because of the flow of Azerbaijani oil, their economy can essentially keep functioning (The Mongolians will probably want a railway, and they might agree to a joint occupation of Tuva to keep the Russians out, since in 1919 it is legally an independent country).
Kazakhstan's biggest distraction really will be organising the Central Asian response to the continuing violence in Afghanistan. They really need to make sure that the allied occupation forces don't collapse and that their gear does not fall into the hands of the Taliban. Expect the British to also be supplying US forces with fuel from the Raj, so they can hang on for at least a while. To be quite frank, the local US forces will probably be confronted with officers of the Raj suggesting they just use poison gas on Taleban strongpoints; the Taleban need to be eliminated as quickly as possible to avoid being a massive threat to the Raj.
Now we move on to areas of the world which are not going to be part of the Russian Revolutionary Wars which may last for the next decade!
Bolivia -- Bolivian politics is a chronic dysfunction of have-nots and national outrage. They will launch the Second War of the Pacific to regain their coastline. It isn't a matter of if. And the downtime Peruvians... Are almost certainly going to join in.
Paraguay -- Here we have another chronically dysfunctional have-not country that is fueled by national outrage and a mythos of the incredible resistance in the War of the Triple Alliance. Are the Allies game for a rematch? I actually rate the chances of this war breaking out immediately to be less, but interruptions of Paraguayan free trade with the wider world could easily result in conflict rapidly escalating over the Parana river. The Paraguayans even have their 1930s vintage river gunboats still for dominating it....
Ethiopia -- The Ethiopian political response to arriving in the age of colonialism (and having a chance to "do it over again" with political unification with Eritrea) is going to be the hammer of Africa. They have the most veteran and best fitted military in all of sub-Saharan Africa, most suited for this kind of war. Are they going to tolerate being surrounded by Colonialism? Even the Egyptians may ask for their help against Britain.
