Will we be able to choose our coop partner, or will it be random? This game is one that i really want to play with my friends, not just who ever the server serves up. Please, tell me that we can pick our coop partner.

As I know, yes. You can choose who you want to play (Caddoc or E'lara) before the game start. In a single-player only game. You can switch between both character anytime you want. In a networking game. After a player join your game, you both have the chance to switch control to another character between stages. This switch-hand is optional. And before a player join your game. You can still switch between characters anytime you want like singleplayer.

*Not 100% sure, so if I'm wrong, please tell.