Thunder Kai, unfortunately for now SaltySD does not allow you to add game files to Smash, it only replaces existing ones. If SaltySD had an update for being able to add Smash files, this would be possible, but for now we have to wait.
The _cXX extrenstion on the nus3bank name determines the slot it uses by default, so adding _c00 to a nus3bank file would help it only work on the first costume slot, while the default voices would work off of the original nus3bank voice file (without the _cXX). Say, if you had Shadow on the 5th slot of Sonic, you can put the modified sonic voice nus3bank as (namehere)_c04.nus3bank, so all your other slots would work with Sonic voices but the one other voice would be Shadow's.
If SaltySD could add files, we'd also be able to have additional costume slots. ui_character_db.bin works exactly the same as the Wii U version.
AND XERO MIKU, I LOVE YOU MAN
I'm on it for those banner/icon files!