You only need 1 copy of a unit to be usable but you're going to need 12 more copies of them or 5star upgrade cards to fully upgrade all their skills. I'm not really sure if that's better. Unless 5star upgrade cards/fragments are really easy to get.

I've used about ~10 upgrade cards between 4 and 5 star ones. If failing a skill upgrade is possible, it hasn't happen to me yet.

I'm going to take a guess that the new gacha girls will eventually be put into regular gacha like how Mononofu did it.