A lot of job have cross skills from other classes. Monk requires lancer to level 15 and pugilist to level 30. But you can also use skills from marauder up to level 12.
Dragoon is a bit different with marauder to level 15, lancer to level 30, but also uses some pugilist skills.

http://www.ffxivguild.com/ff14-job-requirements-arr/
http://www.ffxivguild.com/ff14-cross-class-skill-guide/