The higher one's mechanical or magical aptitude will determine how powerful the buff of the gear they use will be, so you generally want to use the type of gear that fits their aptitude, but neutral gear exists.

For a mage, Harm can be your main damage dealing trick. at 100% aptitude, you'll burn through all trash encounters, which paves the way for Quench Life and Disintegrate, which can be your boss killers. the type of magical gear you use will be dependant on size. I prefer the heavier armor the better, if strength and dex allow, but encounters don't last very long at high level. staves exist, but they're more useful in the inventories of your companions (bug, glitch, exploit, working as intended with the Unofficial Patch? idk), since it'll in effect give them spells to use without mana. healing staves in particular can be life savers. for weapons, you can try for ones that give a secondary buff to things you'll find more useful as a caster, since you can forego using melee or ranged physical attacks all together.

for Sogg, there's large magical leather armor that has a 1% chance of showing up at wise women at high level, which you can kind of scum for, or hope you get lucky in a chest. this is also what I used on my half-ogre mage-y type. in general Sogg will use leather armor. there's a handful of suits of armor for size large characters, but they're ridiculously rare and marginally better, so I would not hold my breath. you can give him a healing staff too. he has very high strength, so you can give him the heavy hitter swords. at max level, Sogg has completely neutral aptitude.

for Virgil, I used neutral plate and a high speed sword and a magical shield. with the before mentioned healing staves. his strength doesn't go very high, so pay attention to if he can weild thigns or not. there's an easily but randomly obtained magical axe that gives a bonus to strength, which I found useful early one. at high level, Virgil has magical aptitude, but I don't remember if it's 100% or lower than that or what

you want the dog. he requires no gear and does not count towards your follower limit, tho there are a few rare instances/quirks where people don't like that you have a mutt with you

Sogg, Virgil, and the dog, are my main dudes in any playthrough of mine. I find the rest of the companions pale in comparison. and they're very easily obtained, except for the dog, which starts an unseen time limit once you enter the map he's on.

that's all I can think of. this is NOT OPTIMAL by any means, and the staves trick maaaaay be an exploit, but this is just what has gotten me through the game multiple times, so I can attest it "works" maybe somebody who is more familiar with the inner workings of the gear can chime in