Tokubetsu said:
You can really do most anything you want with any character and they'll be decent at it. There are minor stat variances that help them at certain things (Ashe and Penelo are better with magic, Balthier and Basch are the strongest, etc.) but they don't make nearly the difference your own progression through the license board will.
My approach was to specialize the characters in a way that would let me evenly distribute quickenings early in the game. You can zoom around the license board and find where the quickenings are, so plan accordingly so that guys don't need to trek across a whole board to get them if possible.
To that end here's what I did with party.
Basch - Paladin type, a little white magic and acessory growth, but primarily used his LPs to fill out the augment and heavy weapons/armor paths on the board. Thanks to him I quickly found the best HP and physical damage boosters, as well as where Katanas, Great Swords, etc. were located.
Balthier - His early start on gun development and technicks lead me to develop him along those two paths. He is now my primary technick guy, but along the way to guns I found poles, which he does better with (imo) thanks to his high strength. As a result I've also branched him into spears (Basch uncovered a very cheap path for him) and he is now a front line poles and spears guy with a little white magic and heavy technick usage. His big tech development obviously makes him one of my stealers, a skill I exploit pretty heavily (Gambit of Foe HP = 100% > Steal) to maximize looting and therefore money.
Vaan - Good all around, I quickly developed him along the black magic path while also picking up a little heavy armor and two extra sword levels. He's my primary black magic caster and now I plan to develop him as a Katana wielder with heavy and mystic armor, making him an offensive force with a little support and healing magic tacked on. He's also a stealer.
Ashe - Much like Vaan, only with white magic. She comes with a big white magic head start, so I just went with it, filled out maces and staves for her to get the three quickenings down there, and have begun her Katana development. Like Ashe, heavy and mystic armor with Katanas currently, but if a powerful staff, rod, measure, or mace comes along I have someone who can use it.
Fran - Already has a good jump on bow devlepment, so I followed it through. Same with black magic. She's one of my long range attackers (which I like to control in boss fights) with some support magic but mostly offensive. I've begun progressing her in Ninja swords as well, as both bows and ninja swords are strength and speed dependent, her two best features from what I can tell.
Penelo - Early development though daggers and ninja swords, which I'm now going to turn into gun usage, as her physical abilities look to be the weakest, making her an ideal gun user (no physical modifiers). Early on she assisted in my exploration of the grid by cover the top section of augments and the light/mystic armor path in armor, so she quickly could pick up the white magics, making her into my second long range but one who uses support and healing magic.
I've since paired them off into two teams of three. Basch, Vaan, and Penelo are one group, while Balthier, Fran, and Ashe are the other. This way Basch and Balthier can share armors, as can Vaan and Ashe, and Fran and Penelo. I only need three sets of great armor at any one time and I swap teams either by dungeon or every 5 levels, which ever comes first. Along with that I have the non-actives equip Gold Amulets which double LP gained, so either the characters are active, racking up XP, or inactive, racking up LP.
I think its working out quite well. By sending guys on specific paths I got to see what all the different armor and weapon types were like and was able to make more informed choices about how I wanted to specialize each character. Obviously everyone dabbles a little in white magic, as its free healing and pretty essential, as well as technicks, since Charge is a really great way to regain MP for your healers or post-Quickening combos in big battles. I'm currently ~40 hours into the game and I've had all three quickenings for everyone for almost half that, and full view of the grid through various characters for ~10-12 hours of play.