All i can say is the game is absolutely brilliant. I say it's easily the best Halo installment so far it doesn't really feel like an expansion to me it felt like a complete game. The storyline keeps you drawn in the gameplay has been made alot more challenging which is fun and the music in the game is also brilliant. I'm giving it a 10/10.
The basic storyline is you play as a ODST trooper (So just a human) and it's set just before the events of halo 3. Basically you're fighting the fight that Masterchief left when he followed the covenant cruiser to the ark.
Not in campaign mode no but it has the same settings for multiplayer and online where you can pick what emblems, colours and what armor you want. Though you need to unlock the majority of the content before you can do it though.