While there are some similarities, I don't think they're close enough to say that FFVII's elements were borrowed from FFVI.
Don't get me wrong, I love FFVI better than FFVII. But I think you might be trying a little too hard to look for similarities between the two.
I also completely disagree with the character comparisons. Just because some have the same job class and Barret has a gun arm while Edgar has guns and tools, that doesn't mean they're similar. In fact, Edgar and Barret have completely different personalities. As are Shadow and Vincent, and Terra and Cloud's personalities, etc. Everyone of those character comparisons are completely wrong.
For example, Barret has a very demanding attitude and he's got a very intimidating appearance, but he has extreme determination to fight for Marlene. On the other hand, Edgar is a womanizer, King of Figaro, and has a knack for tools and technology, and though his ladies' man appearance can be a turnoff to some, he's actually a very caring person. The two have basically nothing in common with each other.
As for the two main protagonist and antagonist comparisons, they're a little more understandable, but their personalities are still a lot different enough to be sure that FFVII copied from FFVI.
Terra is an Esper/Human hybrid. Cloud is a "former SOLDIER". Terra was confused about what love is and about human emotion, and in the end she found out the importance of her friendships and love and stuff like that. Cloud was virtually void of human emotion and had this very apathetic attitude, but he still cared about his friends. They're completely different people.
And Kefka and Sephiroth. Sephiroth was part of an experiment to become the strongest SOLDIER. Kefka was a Magitek Knight and accidentally went insane. But, Sephiroth was more or less misguided, trying to take revenge, while Kefka was already bent on destruction and had no other motive other than just destroying everything, because he believed there was no purpose to life. Also, Sephiroth had various abilities already at his disposal, but Kefka had to continually gain power as the game progressed.
Weapons, Government, Monsters, and Plot Elements are irrelevant, since most Final Fantasy games are like that. Weapons are always frequent recurring elements in most games, the government scenario... there's a lot of different ones. Of course, that doesn't mean they're copying off of FFVI. Monsters, there's always those kinds of monsters. I mean, sure, Ultima Weapon originally came from FFVI, and the WEAPONS in FFVII were based off of that, but that's it. Plot Elements, Most games(if not ALL games) have an antagonist who is trying to wreak havoc on the world... so I don't see why you'd think that is such a significant thing.
I guess the Magicite and Materia are similar, but no, they're also very different too.
So yeah... I don't think they're the same at all.
P.S:Where the heck did you get the idea that Zack and Celes are alike?
