Yep, what Baddo said is basically all true ;D
Even if you don't paticulaly jump several metres in the air at the thought of playing as Finnish soldiers, you will still enjoy it, and learn lots about the Finnish army and weapons.
But in answer to your questions.
1. No, its not a replacement mod. But it does come with a lot of new units. However, it makes the resistance speak Finnish, and, as Baddo said, makes gun sounds much more realistic.
2.
http://kyllikki.tiimipeli.net/addons.php3. The campaign basically follows a Finnish soldier stationed on Nogova. Communist Rebels attack, and after a while, Russia is shown behind the rebellion. So you get shipped off to Finland where you desperatly hold off a huge Russian army attempting to cross the border (I think) . I'll let you find out the rest
4. It is worth downloading. When you play the missions on a snow covered landscape, with such high quality troops, and new music put together it just has this really good feel to the game. Hard to explain ;D