You can develop your characters anyway you like. The characters stats have very little differences. Stats are almost irrelevent in deciding what to do with the characters as Wolf said any character can cast any spell and do it very effectively, any character can do max damage with their best weapons. Heck there are even materia that you can equip to the characters that will boost speed, magic power, hit points, magic points, and luck.

You can also go down to the sunken airship and morph monsters down there for sources thus giving you an unlimited supply of sources if you need to tweak a characters stats to better fill a particular role.

Stats in this game are fairly unimportant, it mostly comes down to materia and weapons and equipment. You are completely free to build them up anyway you want, beautiful isn't it.