BLM also draws from their own aether reserves, thats the entire BLM story arc, and why you need the soulstone to practice advanced thaumaturgy.
As these mage jobs have developed through the series, there's been somewhat of a trend of the 'advanced' jobs being as follows:
Black Mage > Magus
White Mage > Seer
Red Mage > Sage
And with the NPCs having non-typical or advanced jobs, sure, Sage is sn option.
However I believe Louisoix was referred to as a Sage.
I think Y'shtola/Matoya is in fact a Magus.
