arguably the main character is Terra cuz she is directly linked to some major plot elements. but it really could be anyone.

You won't have to worry too much about choosing a party for the first 1/3 of the game or so, as the plot is fairly linear. Once you get a choice, just figure out which guys have abilities that you like, and use them. There are a few points where 1-2 party members are "forced" but you can just fill the remaining slots with your favorites and you'll be fine.

First esper, it kinda depends how fast you play but it's after about 8-10 major plot elements so perhaps 6-8 hours (maybe).