Yeah, for FireFight it's confirmed for local to be two people and for Xbox live, four. However, the Campaign has not been confirmed. As for the characters, in a E3 demo, it showed a "Rookie" waypoint which makes me think that because of this identification, you will be able to play as multiple ODSTs. Ex: "Mickey" waypoint is 26m, "Dutch" waypoint is 78m, etc.
~Hunter