I suggest giving spells that cure statuses or heal to Firion, as his average magic stats, decent HP, and permanent place in the party makes him a good fighter/healer hybrid. Give him Cure, Life, Barrier, Haste, and Wall. That way he can advance his spells throughout the game so they'll be super-high levels. Also, Warp.

Also, Maria should gets spells like Fire, Blizzard, and Thunder as she is always in the part and can level them throughout the game. She also has a respectable magic stat for extra damage. Give her status magic like Stop, Curse, and Stone as well for the same reason.

Guy I'd keep magic-less except for maybe a Cure or Fire in case of emergencies.