Huh. That's actually a question I didn't realise I wanted the answer to until you posed it.

If an authority on the subject already happens to be browsing the thread, could they (briefly) expand on how connected the German communist movement was to those in other nations, especially the USSR?

I'm not an expert on this subject at all, but I have been doing a little research in peacemaking after the First World War so I can provide you to some sources that might point you in the right direction:

Politics and Diplomacy of Peacemaking by A. Mayer.

Recasting Bourgeois Europe by Charles S. Maier.

Revolution in Central Europe 1918-1919 by F. L. Carston

This is what I can say: The Polish state was partly created because of a fear of Bolshevism spreading to Germany. Germany had the second largest communist party associated with Comintern at the time so it's understandable.

Sources for that would be:

The Lights that Failed: European History International History 1919-1933 by R. J. Overy

Versailles After 75 Years (the chapter on Soviet reaction to Versailles in particular.)

Also, I know this wasn't your question, but it's interesting to look at the history of what the Comintern and Lenin thought of Germany at this time. Marxist.org has some good stuff on Lenin, and I do know some good books on the Comintern but I can't remember them right now.

Eugen Debs wrote an article in February 1919 in which he proclaimed his support for the Spartacists and denounced the Social Democratic Party government as reactionary. The piece also eulogizes Luxemburg and Liebknecht.
The article is reproduced here
I'll post a more detailed analysis of it tomorrow.