Red Mage is such a well-rounded main job that it adds very little as a subjob, even for other mage classes. Its primary advantages are the Fast Cast ability, quickening spell casting spell time, and its high intelligence, making it a possible alternative to White Mage for the Black Mage's sub. Despite increased damage, however, the Black Mage/Red Mage will never gain access to Raise, Poisona, Curaga, Divine Seal, and other abilities which make Black Mage/White Mage the most popular choice

heres a couple links to some more viable subjob choices :

http://wiki.ffxiclopedia.org/wiki/Red_Mage_Support_Job
http://wiki.ffxiclopedia.org/wiki/Bl...ge_Support_Job
http://wiki.ffxiclopedia.org/wiki/Scholar_Support_Job
http://wiki.ffxiclopedia.org/wiki/Wh...ge_Support_Job