Spike. How would you distinguish between State Capitalism and Communism or more correctly Socialism? Indeed what are the features of all three systems that distinguish them from a democratic mixed economy or a consitutional monarchy?
State Capitalism - Is where the government controls the economy and produces surplus value through wage labour, but that surplus value is controlled by the bureaucrats not the workers (producers)
Communism - All property is held in common, from each according to their abilities to each according to their needs etc. No wage labour, no exchange based economy. Total personal freedom as long as you're not harming or exploiting others. Another term is Libertarianism, not in the sense it was hijacked by the Libertarian Party, but in the sense meant by the french libertarians in the 19th Century.
Socialism has been debased as a word imo - it can mean anything from Swedish style social democracy to the 'revolutionary politics' of the different trotskyist splinter groups around the world, to the libertarian socialism which has dozens of different interpretations itself.
So 'socialism' in some forms is compatabile with a democratic mixed economy/constitutional monarchy, state capitalism is also compatabile in theory with a liberal democratic state in theory though I can't think of a single example. (maybe Republican Spain, but I doubt it)
Communism is not compatabile with any other system as far as I can see. I'm no expert though.