I often switch roles every time I play FFVII. Last game I had Barret as my primary healer, tifa as my prime magic user, Aerith, Red, and Cid as my attackers, Vincent as my long range and Cloud as my status magic user. It dosn't really matter, especially at level 99, you just crush everything anyway...

You can always switch them around during the game too; it depends on your fighting style, what materia you like to use, and which other characters you have in your party.

As to the Aerith weapon problem, The Parasol is the most powerful, but the Princess Guard is the most practical.