Dissidia Sedecim - A recreation of FF6's combat engine

Well, I can't help you with your project but I do have an idea or two if you want to use 'em... depending on how many characters you intend to make.

Cecil Harvey: If you're going to make him, make him both a Paladin and a Dark Knight simultaneously. Give him Cover and Dark Wave. Have Dark Wave do damage based on his missing HP (difference between Max and Current, possibly modified by Stamina), and make Cover reduce the damage he takes from the blow by half (and possibly cover for Magic as well). That way, he can protect an ally and use that damage to boost his own special attack. Or, if you're feeling ambitious, he could store all the damage he takes in a buffer (regardless of how much healing he receives) and then empty that buffer with Dark Wave, meaning players could heal him and still benefit from the strategy.

Rikku: Don't use Tidus from FFX. As much as I love Tidus, Rikku's basic gameplay is more interesting. Give all the enemies a diminishing list of components to steal from (first 100% chance, then 50%, then 25%, etc.) and then let her mix them to cast various spells. There's that inclination to use Yuna instead for her persistent summons (done rather like the Control menu), but she would diminish the crossover aspects because all the other characters would have to leave the fight while she was summoning. Also, you haven't got any thieves yet... though I suppose Zidane from FF9 could also fill that role.

What you've done so far is very cool and I like the mechanics I'm seeing (that thing with Noctis losing Max HP is just brilliant).