until you got a very good army, you can leave your secondary provinces with a fort 3 and 50 archers (fort 4 if you want, but that's expensive in the beginning) .... anyway, raiders will come at you, no matter what ... but a lot less will come if you have an active army designed for defensive battles (offensive armies are usually 60% swords/40% cavalry).
look, in R2, I've been attacked only 7 times since the beginning of the era, 3 of which have been pop pillages that were not worth ambushing, and 3 were repulsed. All in all, only 1 succeeded for a meagre 11K gold of ressources ... that's nothing for a 1 month period, don't you think ?
and in v4 realm, you are even less likely to be attacked because of the no-offline rule and abundance of goldmines ....
I prefer plains as secondary provinces instead of mountains, because they can produce villagers much more faster, and that's essential in case of Alliance wars to sustain the war efforts while being repeatedly pop pillaged. Good luck trying to grow your pop n a mountain w/forest while being pop pillaged at 20% every 168hrs
as for the differences between mountains and mountains w/forest, it's not so much that one is better than the others IMO, but rather that they provide different unit bonuses which you can use to shape a powerful defensive army. But overall, they are both great ...
Have fun,