Were There Fringe Groups Like Antifa or Brown-Shirts Existing in the 1918 to 1921 Era of Germany

 Yes. Examples:

Deutschvölkischer Schutz- und Trutzbund (1919-1921).  Antisemitic völkisch (folkish) group.  Probably near 200k toward the end....geared against "Jewish influence."  They were dissolved by 1922...by the Weimar Republic.

Black Reichswehr (1919-1920).  Probably near 10k by the peak.  I would call them more of a militia group....a private army of sorts.  Known for carrying out sabotage and assassinations.  They were also dissolved by 1922....by the Weimar Republic.

Spartacus League (1918-1920).  This was a leftist group....anti-war in nature.  Probably peaked at 5,000 members. As they were dissolved....they morphed into KPD, and later into Antifa. (Nifty name, you must admit)

Proletarian Hundreds (1918-1919).  Mostly active in the NW of Germany.  Worked in cells of 100.  They later were dissolved, and blended into KPD, and later into Antifa.

Reichsbanner Schwarz-Rot-Gold (1918-probably throughout most of the 1920s).  SPD paramilitary....some say they numbered near 2-million total.  If you were to be asked if a 'good-guy' militia existed in this era.....this might be the only one....however, they clashed in physical violence with various groups.

If you could count all these groups, and suggested memberships....there's probably 3-to-4 million men....mostly all vets from the war.   Instead of fighting the French, British or Americans....they were mostly fighting amongst themselves.

The idea that the Brown-Shirts or Antifa were the 'start-ups'?  NO.....dozens of groups were starting up in late 1918, and carried through to the Brown-Shirt/Antifa era.