So you have a Fighter, White Mage, Black Mage, and a Thief? You need a Red Mage to complete the set, probably.

The Red Mage would be able to do some of everything (fighting, healing, magic attack) but not really specialized. It's pretty much Kimahri, except Kimahri never really had access to everything at once (which is why he's generally considered useless - if you make him a fighter, he's not as good as Auron, if you want another healer, he won't be as good as Yuna, etc. ). A 'real' Red Mage doesn't just have all the paths open to him from the start, he takes all of them at once, so he could be a good attacker and heal a bit and do some random Bolt 2 spells.

As to #6, Disposable Unimportant Gimmick Character is a good choice (see FF6's Mog, FF7's Cait Sith, FF9's Quina, 50% of Chrono Cross's cast, and FF10's Kimahri). Either that or you could follow the FF2j example of "put random people in that slot and kill them off constantly".