I'm not sure that the Russians would want Germany in the USSR,
This was actually a subject of debate between Lenin and Stalin in 1920, when Poland seemed to be facing defeat, and Germany was seen to be the next domino. As Robert Service writes in
Stalin: A Biography:
"Stalin and Lenin also undertook preliminary planning for the kind of Europe they expected to organise when socialist seizures of power took place. Their grandiose visions take the breath away. Before the Second Comintern Congress, Lenin urged the need for a general federation including Germany, and he made clear that he wanted the economy of such a federation to be ‘administered from a single organ’. Stalin rejected this as impractical:
"If you think you’d ever get Germany to enter a federation with the same rights as Ukraine, you are mistaken. If you think that even Poland, which has been constituted as a bourgeois state with all its attributes, would enter the Union with the same rights as Ukraine you are mistaken.
"Lenin was angry. The implication of Stalin’s comment was that considerations of national pride would impel Russia and Germany to remain separate states for the foreseeable future. Lenin sent him a ‘threatening letter’ which charged him with chauvinism.27 It was Lenin’s objective to set up a Union of Soviet Republics of Europe and Asia. His vision of ‘European socialist revolution’ was unchanged since 1917. But Stalin held his ground. The Politburo had to acknowledge the realities of nationhood if the spread of socialism in Europe was to be a success.
"These discussions were hypothetical since the Red Army had not yet reached Poland, far less set up a revolutionary government in Warsaw..."
http://www.rulit.me/books/stalin-a-biography-read-280295-56.html
For a slightly different translation of what Stalin said in 1923 see J. Smith,
The Bolsheviks and the National Question, 1917–23:
"I said [to Lenin] - and this is all preserved in the archives of the Central Committee – that that would not work. If you think the nationalities of former Russia will stay in a framework of federalisation – that is understandable enough, but if you think that Germany will at some point come to you to join a federation with the same rights as Ukraine – you are mistaken. If you think that even Poland, which has taken the form of a bourgeois state with all its attributes, will enter into the composition of a union with the same rights as Ukraine – you are mistaken. That is what I said then. And comrade Lenin sent out a long letter - that is chauvinism , nationalism, we need a centralized world economy, run from a single organ."
Smith adds "Stalin's account of his own letter is accurate, so it is reasonable to suppose that his summary of Lenin's reply, which may also have been known to members of his audience, is equally correct."
On the next page, Smith summarizes: "If Stalin's 1923 speech is accurate, then, Lenin had accused him of chauvinism and nationalism for denying that Germany and Poland could be treated on the same basis as the Ukraine. Thus in 1920 Lenin wanted a union of
all Soviet republics, including those yet to be built, while Stalin did not believe this was possible – Lenin was the centraliser, Stalin the separatist - the very opposite of [the traditional interpretation]..."
https://books.google.com/books?id=pSZ9DAAAQBAJ&pg=PA179