Heavy Inf Scale means heavy infantry wearing scale mail I gather.
The numbers are, I think, supposed to make all groups roughly equally powerful. So 354 militia would be the equivalent of 100 Heavy Infantry, or 81 Vine Ogre. This does not include morale however (I believe), so some of the groups will break in an actual battle, while others will stick to the death.
The Power and Overall numbers mean... Well, something Saber Cherry explained, but nothing my limited grasp of maths could make sense of.
