I'm not sure how much weaker Caster Gil is- he doesn't seem to have Enuma Elish. What really makes Gil broken in any form though is that he has basically any Noble Phantasm you can think of, so he could pull out magic wands that inflict wounds that never heal, or cause instant death or whatever.

And yeah I'd like to know how good we should be too. It says high stats are okay but broken abilities aren't, so are really high-class brute force guys like Arthur, Achilles, and Sigurd acceptable?