Speaking of, since everyone's using Materia of some kind, should I start Leo off with higher-level stuff (i.e. Fira, Blizzara, Thundara), or just imply he can use the same spells more effectively?
He'll get more materia slots, and due to his training, have more skill casting them. The spells will be a little more powerful (though a Fira from one of the others will still be more powerful than a Fire from him), and he'll have more stamina when casting. (Basically, he gets more Materia Slots, a higher MAG, and a higher Max MP :U)
Melee: Med MAG, Low MP, 3-4 Materia Slots (2 linked)
Marksman: Med MAG, Low MP, 3-4 Materia Slots (2 linked)
Demo: Low MAG, Med MP, 3-4 Materia Slots (2 Linked)
Hacker: High MAG, Low MP, 4-5 Materia Slots (2 Linked)
Heavy: Low MAG, Low MP, 2-3 Materia Slots (2 Linked)
Mighty Grunt: Low MAG, Low MP, 2-3 Materia Slots (0 Linked)
Medic: High MAG, Med MP, 4-5 Materia Slots (4 linked)
Mage: High MAG, High MP, 8 Materia Slots (6 Linked)
Command: Med MAG, Med MP, 4 Materia Slots (4 Linked)
I put a little variation in most of them, so its not too restrictive. This IS freeform, and not system after all. I just think sticking to those parameters will help keep things relatively balanced.
Simply put, Low MAG means your spells will be weaker than average, Demo has Low Mag but Med MP because I can imagine there would be uses for magic in demolitions (set off explosives with a fire spell, for example) but they wouldn't generally be casting said spells AT enemies, so wouldn't be trained to deal heavy direct damage with them.
Low MP means you'll be tired out by casting sooner. Melee and Marksman have Low MP but Med MAG because they'd be trained to rely more on their weapons, but would be given SOME training on utilizing magic in a pinch, so they'd be trained to make what little magic they cast count.
Heavy and Mighty Grunt are both... well... pitiful with magic, because they spend so much time focusing on their gear. Because the Heavy is expected to fight on foot, while the Mighty Grunt isn't, however, they are given two linked slots, while the Mighty Grunt isn't. Linked slots, in case you don't remember from FF7, allow materia to be linked with support materia for added effects (HP Absorb, MP Absorb, Quadra Magic, All, MP Turbo, etc.)
Medic has High MAG and Med MP, because they have to use restorative materia as part of injury treatment. However, being primarily support in that regard, and not offensive, they're not expected to be throwing magic around constantly, and thus don't have the same stamina as Mage does for it.
Mage has high MAG, MP and the most materia slots, and the most linked, making them the most skilled with magic in all regards.
Command is balanced as they're expected to be adaptable, and capable of leading by example. They can't hold a candle to Mage or Medic, but are better at it than the others. In exchange, Command also lacks the specialized skills of each other others, or their gear. As a result, Command is basically a jack-of-all-trades type character, able to fill in for the others until they can get back on their feet.
I'll also post up a list of materia shortly. Any questions or comments, suggestions to any lapse in logic I may have had?EDIT:
Hacker has high MAG, but Low MP, as they are not intended to engage in prolonged direct combat. They do train to hit hard, however, that way, when they MUST use a spell, they make it count. Thunder spells could also be used to disrupt electronics, and aid in their hacking, status spells would also assist in swiftly incapacitating.