A Democracy from my experience cannot win against a communist government. A republic will give a person a few extra turns to try to make something happen but if you have 2 equally skilled players face off ne using communism the other using repub or demo the communist will always win UNLESS the technology gap between the two becomes substantial. Thats why you have to use your overwhelmingly strong military to damage your opponent constantly this will even the odds out between your fairly poor science research ability and your opponents science research since he will be under non stop assault he will
1) not have time to build structures but you will since you can draft many citizens and still have military police.
2) Lose cities and thus lose science research and money
3) Will eventually get into major war weariness at which point functioning becomes impossible.
I won't admit to being an expert on communism because I'm not. But I'm sure someone somewhere out there really has a grasp on this type of government to the point where they can be more effective then any other government type out there. It would be nice if one of those people could also post there experiences with communism.