Unless it was modelled directly to the back, you could simply use Kimera 0.95 to remove it, or failing that, you'd just have to delete the model from the back manually, that way, when he draws the sword, It won't have the same sword on his back as he does in his hand.
for the other models, you can quite happily keep the sword on his back if you so wish.
As for making that sword a buster sword replacement, it seems a logical choice since he's carrying it all the time.
But, if you're talking about making the battle model with a sword on his back, that will mess up some the logical side of quite a few default animations Cloud has, such as the magic, summon and victory animations in which he puts his sword on his back, and if there's already a sword, it'll cause clipping and would look odd.
I'd suggest having a sword on the back in the field, expect for when he's wielding it, and in battle, no sword on his back, but the sword in question can be the Buster Sword.