Have you seen the March 1918 scenario thread? We'd looked at various options for the Finnish civil war, including giving Soviet Russia control of some southern Finnish provinces to represent the Reds. However, since historically it was over within a couple of months of the scenario start, and the AI is unlikely to manage the same thing, I was currently leaning towards just giving the country FIN a high level of dissent at the start of the scenario, to represent the civil war. However, if you're interested in developing this in more detail, we'll be glad to look at your ideas.