The any ISW basically requires a GM, so that's who would be running the OpFor. In fact my viewpoint is that of the GM, I asked previously about this sort of thing (actually on extra powers, even finding the 6 people needed to run the basic ISW in IO is unlikely) and the advice I received was to ignore them/remove them from the map, which REALLY doesn't work for a 1st Succession War game, what happened in canon is that they fought over the Hegemony, so simply removing those system can't work.
But yes, before play starts is a good idea, or if you need to change things let everyone know at the same time.