Questions about the gameplay of Heroes 6.

Thank you, you helped me figure it out! The interface is quite confusing, though. Regarding skills... I don't fully understand the skills yet. And I've gathered that you can't learn everything; you have to choose which skills to learn. I think you don't need the Mage Guild to learn magic skills, right? And another question: the first campaign was very short, only two maps. The other campaigns were all unlocked immediately, but I haven't started playing them yet. Does the order of the campaigns matter? And has anyone played as Necromancers? Do they, like in Heroes 5, build an army from corpses?