I still tend to hold Janus in higher esteem -- I'd trust Melchior a lot more than some Zealian jerk. He probably was a product of his circumstances, though. However, I still believe that Janus could master any magical technique invented by ZEAL, as his research is likely at the root of them. He's not in the story, though, so that's pure speculation on my part.
For example, my writing -- before eighth grade, I didn't have a damn clue how to write anything. Ever. And then that year comes, and all of a sudden that's what I want to do for a passion. My entire class made promise to write, no matter what else I do. The talent (or what I hope is talent) sprouted late, but it's turned out quite amazingly both as simple writing and for a career as a physicist, just going by what I've been told by the people who read my stuff and give me my grades.
Janus was definitely a late bloomer, but who cares? He could still set this whole county on fire.
As for the levels, potential is still there for any character class, but I'm still not sure where Lavos, FATE, and the Dragon God fit. Does the level you can reach depend on innate potential? I suppose that because of 13,000 years of extra magical genes, the ZEAL residents would have more magical potential than the average human, but in the Keystone timeline, allowing for the destruction of Ayla's tribe and the accepted evolutionary record, not to mention the lore about the Frozen Flame, it took us a few million years to get to where we are now in terms of potential -- a mere 13,000 isn't going to make that much of a difference, unless Lavos' presence is a lot more powerful than we thought.
And the yelling thing...just...no. Chanting, yes. But yelling, despite the obvious similarity in character design, is too DBZ-ish.