Sweboz units get a big bonus in woods, I dont think it will matter too much which units you actually use if fighting Romans in a forest...

About the skirmishers, I found them to be a great value for money unit. When the enemy has been weakened or tired then skirmishers can even fight the enemy in hand to hand, especially in the woods... At the start I had as many skirmishers as line infantry, later I dropped to 1/3.

I would prefer to use better troops, but until the economy is very strong having skirmishers is a good way to bulk up the army without going broke. Also its more historical :)