Unlike in Red Alert, why would Stalin attack the rest of Europe? He had been advocating for 'Socialism in One Country' instead of 'Permanent Revolution' since 1924.
Really, "socialism in one country" has nothing to do with it.
"Socialism in one country" was an attempt to keep up the morale of the Bolsheviks after the failure of Europe to undergo successful Communist revolutions in 1918-23. If did
not mean that the USSR would give up on encouraging revolutions abroad, only that as of circa 1925 there seemed little immediate chance that such revolutions would succeed (at least in Europe) and in the meantime the USSR could build socialism even without such revolutions.
I agree with Leszek Kolakowski, *Main Currents of Marxism*, "It is possible that if Trotsky had been in charge of Soviet foreign policy and the Comintern in the 1920s he would have taken more interest than Stalin did in organizing Communist risings abroad, but there is no reason to think his efforts would have had any success. Naturally he used every defeat of Communists in the world to accuse Stalin of neglecting the revolutionary cause. But it is not at all clear what Stalin could have done if he had been actuated by the internationalist zeal which Trotsky accused him of lacking. Russia had no no means of ensuring a German Communist victory in 1923 or a Chinese one in 1926. Trotsky's later charge that the Comintern failed to exploit revolutionary opportunities because of Stalin's doctrine of socialism in one country is completely devoid of substance."
https://books.google.com/books?id=qUCxpznbkaoC&pg=PA807
Furthermore, even if we assume that Trotsky was more anxious than Stalin to encourage revolutions abroad, that is not the same thing as invading foreign countries with the Red Army. On that, Trotsky seems to have been if anything a bit more cautious than Stalin. Trotsky seems to have been at first reluctant to cross the Curzon Line and invade ethnic Poland in 1920. (Some people have questioned this, but Richard Pipes, not exactly an admirer of Trotsky, has defended him on this point: "Several historians have questioned whether Trotsky really opposed the invasion of Poland as he later claimed...But the documents cited against him date from August 1920, when the matter had long since been decided, and Trotsky, having fallen in line like a good Bolshevik, naturally desired a quick and decisive victory."
Russia under the Bolshevik Regime, pp. 182-3.) In 1940 he was somewhat critical of Stalin's military expansion, remarking " Robespierre said that people do not like missionaries with bayonets. Naturally that does not exclude the right and duty to give military aid from without to peoples rebelling against oppression. For example in 1919 when the Entente strangled the Hungarian revolution, we naturally had the right to help Hungary by military measures. This aid would have been understood and justified by the laboring masses of the world. Unfortunately we were too weak ... At present the Kremlin is much stronger from a military point of view. However, it has lost the confidence of the masses both inside the country and abroad..."
https://www.marxists.org/archive/trotsky/1940/xx/ww2.htm (Admittedly, once in exile Trotsky had an incentive to criticize almost everything Stalin did, so this is not proof that he would use the Red Army abroad less than Stalin. But there is no good reason to think he would have used it more.)
To get back to Stalin in a world with a non-Nazi Germany, the first thing to remember is that Germany would have remilitarized even without Hitler. The Allies had already agreed to the principle of military equality for Germany before Hitler came to power. Just what that meant was not entirely clear, but Schleicher was already planning to move toward universal conscription by creating a compulsory militia. The restrictions on the militarization of the Rhineland could also have been overcome without taking even the minimal risk of war Hitler took in OTL; see my post at
https://groups.google.com/d/msg/soc.history.what-if/qtvUxyKQ6hI/tVfsLI6cbMUJ
So a war with Germany will be far more dangerous for Stalin than even in OTL because the Germany of this ATL (which may be semi-military and somewhat authoritarian but not Nazi) will not only be remilitarized but will probably have much better relations with the Western Allies than the Nazis did. There might still be disagreements, but France and the UK (and, though it would presumably confine itself to economic aid) the US would not sit by idly if the USSR invaded Germany. I doubt that Stalin would want to challenge such a combination militarily.
