All, coming to this late. In 1918-19 there were SEVENTEEN revolutions, Soviets established, or service mutinies in Germany, of which the three emntioned above were just the longest lasting or most well-known. As for why the Freikorps formed - the German Army had NO authority to do anything except eat and breathe until the formal signing of the Treay of Versailles in 1919, a bit like the Fleet sitting "interned" not truly prisoners in Scapa Flow. So the government of Germany had no unified military or even paramilitary force to call upon during that period. But wht you DID have were hundreds of thousands of German citizens VERY recently under arms, trained in the use of arms....and the majority of army personnell took the position that it was communist agitation at home in 1917 -18 that lost them the war. So of course they were naturally, the majority of veterans, anti-communist or anti-socialist.
As for communists in the Reichswehr? A bit like ANY army retrenching after a major conflict to peacetime levels - or in the chase of the Reichwehr vastly smaller - it would have been the officer cadre that selected what veterans under arms would remain in the new 100,000 strong army, and i'm willing to bet that THEY ensured no communists, or actually any agitators of ANY kind - remained in the the ranks. Knowing the German Junkers class that made up the army officer class, THEY were the agitators as the Army ALWAYS dabbled in right wing politics in Germany.