Most people I know tend to make Gus this beefy fighter, and that´s understandable, since he´s got the highest potential.
However, I usually want to have the fighter at the top, including two wizards of different kinds.
This meant I had to use Firion as a fighter, since he´s at the top, and therefore the others became two different wizards.
I always play this way; Firion as a Knight, Maria as a White Wizard and Gus as a Black Wizard - you know, the "classic" line-up.
It simply feels better, in the end of the game I´ll have two Knights in the outer areas of the line-up, and the weak wizards "covered" in the middle (well, not really, but the SEEM covered).
I never liked the idea of making Firion a "Paladin", with sword skills and white magic or something, I wanted Firion to co-work with Leon as a decent Knight, it looked cooler and better.