The communism thing is due to a variable at the beginning of rules.txt. That specifies how far communist cities would have an effect of from the capital.
I'm making a modern day scenario, and I just happened to notice that putting an airport in cities altered their trade slightly. Does anybody know the specifics of this? No official documentation seems to comment on this.
Also, I've noticed that sometimes in a Communism certain cities experience corruption. How does this work?