The best way to practice wouldn't to do Zerg campaigns, do the challenges with the Zerg units first, then do practice games against the computer on easy, and if you think you completely creamed him, go up, and so forth.
Not yet. Legacy of the Confederation's newest episode will have a StarCraft II zerg campaign. I've heard of no others.
You don't play the Wings of Liberty campaign to practice Terran. Just play the challenges, as well as custom games vs the AI.
the insane AI can roll over experienced players, so working your way up from the easiest difficulty to hard will make you a decent player. you could also look at some of the commentator's replays to learn the basics of the game and such. I like day9 personally, but there is also HD/Husky's stuff. Also, it helps to look back at your own replays and see if you can figure out where you went wrong. (also, try to scout and make counters to what you see your enemy getting)
