I'm not too certain about the job abilities. Mainly Dragoon is used for attacking, and Paladin is used for Tanking (taking hits for the mages).

You can only start out with the primary jobs (meaning no Dragoon at first) then you have to a quest at level 18 to get a subjob, then you need to do certain quests at level 30+ to get advanced jobs (such as Dragoon and Paladin).

Hope that answers some questions.