Short and sweet, I think Chemist would be better (for my plans) for the base job for generics and the job that special jobs replace. Can Chemist be swapped with Squire for that purpose? If so, how would one go about doing so? I have not seen this answered anywhere yet.
In a very roundabout fashion, yes.
Make the current chemist identical to squire in every way and vice versa (flag their attached skillsets as <Default> and <Item>, respectively) -- just keep squire with basic skill, but swap Basic Skill to contain all of item and vice versa. Swap them in shishi and fiddle around with tactext and you should be golden.
Direct stuff to change the line of code from job 4B to 4C as the base class? Still unknown, I think. (numbers pulled from ass, btw)
Doesn't changing the item skillset cause glitches for generics, like only knowing what the squire knows or something?