I used my (SPOILER)GameShark to erase my Vincent and replace him
with Sephiroth. I have a couple questions about this though
because I haven't done it in a long time.

Is Sephiroth the same level as the character you erased him with or
is he Level 34-ish (SPOILER)like in the Flashbacks?(The GameShark retreives Sephiroth from here and replaces the character with that version.)

-and, Can you equip him with any materia?

-

What I did with Sephiroth was try to beat Emerald with just him. The downside is that hes not controllable. So he was easily merked. My memory is foggy, but I was able to defeat him with just two characters.
I thought about it and said that he must have been set to his default Level then.