Well SA would certainly not be my faction of choice but if I was to do the period I‘d include them as frankly I don‘t think the Reds behaved any better in regards to their political enemies. To my eyes it‘s only what happened after 1933 with the Nazis actually taking over power in Germany what made the difference in the end. Just to play the devils advocate: If the Reds had won that power struggle and errected their Gulags and what have you, maybe we‘d today sit here and ask ourselves if we could justify it to ourselves to play the Rotfrontkämpferbund?
I feel a little disclaimer is in order here:
I certainly don‘t intend to belittle the abhorrent crimes to humanity perpetrated by the Nazis and their henchmen. But at this point in the timeline these deeds are still in the future and I don‘t think most German, let alone foreign, contemporaries really thought it possible that Hitler and his chronies really could pull off the things he‘d laid out in „Mein Kampf“.
Thus I think it‘d be wrong to exclude the SA, which were an essential part of what was going on back then. Actually unlike the SS in WW2, which on the battlefield (as that’s still what we’re recreating on our tables), didn’t do anything other Wehrmacht units couldn’t have performed.
For Babylon Berlin though I actually think if one was to play the period you need to factor in the SA to fully understand the going ons back then.
I can perfectly understand if one is hesitant about giving those bastards their „place in the sun“, but then maybe the Revolution of 1919 itself is probably better suited.