If you're looking for maximum abuse with >Magick Frenzy, Seer as a main isn't the way to go: Books are not the best for dealing damage. A better way to go is to make Seer the secondary class and get ^Dual Wield (from Ragetsu-denbu:Ninja), then using a class that has one-handed edged weapons for you to get two physical hits in on each enemy you target with >Magick Frenzy. Parivir/Seer works better there, but Paladin/Seer has better weapons and armor. Alternatively, you could use Illusionist/Seer and use two Heretic Rods (or Bomb Arms, if your opponents absorb Dark-elem hits). Illusions hit every enemy character, so using Illusion Frenzy with ^Dual Wield gets three hits on every enemy character.
I'm not quite seeing the value of Heritor/Fighter. It's all right, but I don't know how you use her. Enlighten me?
Cid's good. >Ultima Sword in a Trickster's hands is very good, if kinda monotonous.
There are two very different ways to play an Assassin, depending on whether you wish to be a melee fighter and actually take advantage of the rest of the Assassin skills, or be ranged and primarily spam >Ultima Masher. I'm assuming from the Sniper back-up, you're using Penelo ranged. The thing is that with ^Halve MP and MP Channeling, there's not really a need for the Sniper skills, as powerful as they are, because you can use >Ultima Masher every turn. I'd suggest White Mage as support for >Refresh, which clears statuses that your male teammates can pick up that Orb of Minwu can't handle.
Seeq as Viking? A Seeq's Magick stats are generally pretty atrocious, >Tsunami is unusable half the time, and >Pillage doesn't have that great of a success rate of stealing armor and shields without Sleep or Petrify backup. A far more consistent way of dealing damage is the Ranger's Mirror X-Potions, which might require some Gil investment, but Gil isn't incredibly hard to come across. I'd say skip Viking and put Ranger with a Malbow as the main class. For a secondary, I'd suggest Items if you want to take advantage of Eureka Crystals and Grimoire Stones (also not hard to come by) or Lanista. Oh, and make sure to equip him with a Malbow, that will give him some range to work with.
Red Mage/Summoner is better than Summoner/Red Mage for a few reasons. Windsong Rapier has a Magick bonus, better Attack, and better HP being the main ones.
Also, I'm surprised you don't have a Juggler on your team. You can go without one, but >Smile Toss is quite an advantage coupled with a Juggler's high Speed. (You can also combine that with Moogle Knight's >Ultima Charge to have another excuse to see the blue nuke.) --- Stupid men are often capable of things the clever would not dare to contemplate. ~Terry Pratchett
I usually take advantage of Vaan's sky pirate class. You should try using him sometime --- Kaine: "Or maybe I'll put you in the goddamn furnace! How can someone with such a big, smart brain get hypnotized like a little *****? Huh?"