You don't need 6stars to do any of the content in the game right now, they're mostly just for looks. 5star teams can complete everything and after playing for a few months I have enough fivestars to fill every team (but still need to level half of them, rip).

I got my only 6star by cashing on the black friday deal. If you want 6-stars any other way you'll have to, on average, drop 900 flower gems per 6star on 10+1 pulls. While I'm certain people have done that, I wouldn't hold my breath waiting for it to happen.