Need some help understand the class/job system

[Garlyle]

4. Can you switch between those classes freely once you've become a black mage?

You can; the difference between being in Thaumaturge and being in Black Mage is literally whether or not you have the Black Mage soul stone equipped! With that said, the further you go into Black Mage, the more skills you will receive that are completely unique to Black Mage and can't be used in Thaumaturge, and the more important those will become to your capabilities. Before long, you won't have any reason to swap back into Thaumaturge, really.

Thank you both for you're answers they were very helpful, but they did spark a new question, are all abilities cross class or is it only specific ones?

Specific abilities are cross-classable. For instance, you can cross-class the Thaumaturge's Surecast onto other classes... but you can't cross-class Thunder, or Manawall, or Apocastasis, or Fire 3... etc etc

It's also made slightly more complex that Jobs have less cross-class options available to them than classes. For instance, when you're in Black Mage, you won't be able to use, say, Marauder's Foresight or Pugilist's Second Wind, although a Thaumaturge would be able to. However, to be clear, the value of the Job-only abilities vastly outstrips the power of the lost cross-class options, especially since you will generally still be able to use the most valueable ones to your chosen class (eg Archer's Raging Strikes can still be equipped in Black Mage).

Shougun was kind enough to list the skills that Black Mage will be able to take from cross-classing.