Actually, as a lancer, you gain cross-class skills every 5 levels. So, at level 29, you have 5 cross-class skills available.

As a Dragoon, you gain cross-class skills every 10 levels. So at level 29, you have 2 cross-class skills available. In return however, you get better base attributes and gear and unique abilities. You can always change to a base class when going in to a lower level dungeon. Nothing stops you from doing that.