I really hope jobs are no longer tied to classes. So even if you get Arcanist to 50 via SCH, once you equip the SMN job stone you should go back to job lvl 30, the "default" level. Like equiping a job stone puts you in a lvl sync. Otherwise it will be like SMN and SCH are just different "stances" you can use and switch with Arcanist.
About the branching jobs, I think it is safe to assume each base class will branch in 2. Otherwise it seems unfair that Arcanist has the advantage of 2 jobs and poor Archers are stuck with Bard. I didn't play 1.0 after jobs, but did the bard bows all had harps in them? They could adapt minor alterations to weapons to adjust them without breaking the Armoury System, like a G.Axe that looks like a Scythe (Regular Scythes are odd weapons anyway) or making Templar use Halberds only instead of all kinds of lances. I still think that Katanas can easily fit with a great sword using class that takes a oriental job, the way monk did, and also taking a eastern fell with dark knight using great swords. It actually seems kinda wasteful to have Great Sword and Katanas as different classes, the same goes with knifes and kunais for Thief/Ninja, it should be the same class as well.
The major problem I see in that is that classes like GLD are so focused on being tank that it will be strange to branch into something that isn't tankish like Red Mage. Maybe if some skills are replaced when you change to a different job, but it seems easier to just add a new class.