It's really not that hard. Give Tifa the healing materia. Give Yuffie things like throw and steal. Give Cloud things like HP up and the attack all materia (I forget what this one is called?) Give Cait Sith things like Sleepel, Silence, Toad, Mini. Give Vincent things like Haste, Protect, Wall etc. Just adjust the materia so they each have a role. Support/healer/tank what have you. I tweaked it a bit in VII though so that they each also had an element assigned to them. Red XIII was my primary 'black mage' but besides that they each had one black magic materia based off of an element. Berret had Lightning for example. Tifa had Ice.