this is more of a client-side problem
its coded in the client exe that it should read the palette to its speific palette folder location
so if you know the whole coding shenanigans of the client date you are using then go ahead and edit it
basically yes, you could use the original palette, even the classic custom palettes but it won't look good for the alternate sprites
because jRO used a different palette pattern for alternate 3rd job sprites
if you don't get what i mean i think this image would explain much
the first image is the original gx palette next to it is the gx sprite
notice middle image the alternate gx palette and the alt sprite
on the bottom image i used the original gx palette on the alternate sprite
and viola its a DIIIIZZAAZZTAAHHHH!!
and the example is just for the original palette what more when you use the custom palettes that is not dedicated to the alternate sprites