They did create a fog of war system within the grand campaign, as well as a delayed communication system. So that's how you would use scouts (to negate those issues)

Irregular units would just have to be special units that you can only recruit specific numbers of, and can't really build into a tech tree. Something like: privateer units from South Carolina, or "Mountain Men" light infantry from upstate New York. Those kinds of things. If you really wanted to flesh it out, you could institute a rule that you could recruit more or less of them depending on the provinces status.

So, for example, you could recruit crack infantry "1st Rhode Island Militia" unit if you funded Rhode Island well enough. You could recruit multiple "Mountain Men" units from NY depending on how high the population was but also how low the urbanization was. You could recruit more privateers from South Carolina depending on how many trade routes were going through it, etc. Or maybe "Rabel Units" from territories already occupied by enemy forces, that only appear when conquest was recent and insurgent attitudes are high. Things like that. Maybe you could get riflemen in a province if it's mostly rural, but also gets something like a high equipment score from trade or having metallurgy facilities in it. The point is that your base units "line infantry" can all be built and trained the same way, but irregulars have to have a whole different process. Typically, they'd be variant on season and agitation.