amendment to capt krunch's statement about rostering:
rostering is a mafia and nothing else - talked to a guy, who called jeppesen personally about the bidding system; they told him, he got 100% of his bits, on his roster was not even 40% - and yes, they change your roster, how they like it and believe me - you will be legal for your flight.
about the leave - if they cancel your leave for operational reasons, your days will be still there, but you have to fight for it or to get the money at least
good luck