Wow, go away for the weekend and the forums go crazy.
Alright, to answer some questions, all units in the game level up; workers, melee, ranged, everyone. This really comes into play when you use MMORTS, and your army is persistant, because you will only get more and more powerful with each victory. However this doesn't mean you can use your power to pick on the little guys, you will be pitted against armies equal to your own.
We do have heroes in the singleplayer campaigns, who are key characters to the storyline, it would be kind of silly not to have them, and have random "Swordsman 26" be the main character or something.
Spellcasters work differently than any other unit in the game, you will generally have one or two spellcasters per army, and they will take a long time to train, however magic is VERY powerful, a single spell can destroy tons of units.