I can't answer anything about the GBA version cause I've only really played the PS Origins version but I can try to answer your questions.

In terms of what armor to wear, it is important to be concerned about the Spirit/Int penalties but Int seems to have steeper penalties than Spirit. You also have to put in account magic level which will allow you to get around this. Cure and Raise will most likely have naturally high levels (or should at least) and you don't need really high levels for Buff abilities like Hast and Shield since levels only really affect accuracy of the spell working on a target, so level 5 is more than enough to get these abilities to work. You're real issue will be with offensive spells like Warp, Holy, and Ultima which will need higher levels if you wear heavier armor but to be honest, if you have a dedicated black mage, you won't need these spells.

You're black mage should stick to light clothing and robes. Staffs are good as well as shields to build up evasion and speed but you'll eventually want o move them into the back row and drop the shields. A "Paladin" style character can be equipped with whatever you need to make them work, as long as spell levels are high. I wouldn't really recommend teaching more than one character elemental spells. Magic is a bitch to level up so its more practical and better for your sanity to stick to a single character using all the spells. I also recommend you're "Black Mage" learn not only all three elemental magics but Frog (which is actually an instant death spell in this game) Teleport, and Flare. The one spell I do recommend teaching to other people is Cure. You'll need extra so your White Mage/Paladin doesn't blow all their MP in the games rather lengthy dungeons.

As for the fourth character slots...

Minwu - is pretty awesome as is. You really don't need to tinker with him.

Josef - Starts off weak, but don't be fooled, once he gains some levels in his Bare Hand he's a beast.

Gordon - Is pretty crummy but works pretty well with a shield and spear.

Leila - Doesn't really have any special area of expertise, she's kind of all over the place so try to make her more focused.

Gareth - Is a pretty decent warrior but arrives so late in the game he'll be pretty crappy compared to the rest of your party.

????? - Starts off with much higher stats than usual for the fourth character, he already has high proficiency with some weapons and Black Magic. My main recommendation for him is that since its difficult to power level the fourth member of your party, you're best bet is to just give them the best weapons and equipment available to bring them up to par with your other characters.