Action & Traits -> Additional -> Move the pointer to the class you want to take skills from -> check the box so on the skills you want so that it appears at the 'active bar' at the bottom of the window -> move the pointer to the ability -> press square to then activate the "hot-bar palette" feature -> press the combination you want the skill/ability to use.

As for what skills are available, I don't know. You only get to choose a few of them from each class though.
Jobs are only allowed to cross-class up to 5 abilities from different classes where a normal base class can use up to 10.