So, in order to read and understand how skills work, you have to get them 1 at a time or it's just too much?
To use the new jobs, a player will have to have completed up to the 2.55 story which requires a level 50 class/job at least. By this point, if you can't understand how to read tooltips on skills and work out (or even ask) how they could work together and then play test them in an early dungeon, leve or FATE, I'm not sure what to say.
When I first started ARR I had 4 jobs at level 50. And for a good part, each Job plays a little different to how it did in 1.0, not to mention the battle system is a good bit different. It took a little time to figure things out and get familiar with them, but it was by no means difficult to do. It just required a bit of practice and thought.
This is a good middle ground for those who really couldn't care for traditional JRPG levelling at all (it is a fairly dated thing these days) and would like to start from max level and those who do like the level/progress feeling, without the other feeling as though they haven't been catered to at all.
No, it's not.
On their way from level 1 to 50 on their first class, they will learn how the game works and experienced the story and content pre-50 (and likely a good deal post-50 things).
When they then start on one of the 3 new Jobs, they have already done that content 1-30 on their other class choice. So none of that content will be new for them. They should already have grasped the basics of the game by now.
The only real difference is that a person who has never levelled a class of another role type will be able jump straight to level 30 in the role and may find themselves a little lost. That's fair enough. But they can just queue for Sastasha or Tam-Tara if they want to get some early dungeon practice. They shouldn't need to level from 1-15 to get an idea of what their skills do. Reading tooltips isn't a hard task, and they should understand the concepts enough to get an idea of how the class plays up to level 15. They can then proceed to go through dungeons in order as their level allows.