About the 55x Gacha thing:
Yes there was a thing like that. You do 55 pulls and you get the featured Rainbow at the end of those pulls. If you get lucky you might get one from the pulls and a dupe to buff her up immedieatley.
So far this deal happened only 2, with a very long downtime in between each other. So having those spare 250 stashed away for this is a decent idea, though the other deals that give this and that are also a decent pick.

However we had the 55x one not so long ago, so I`d wager we wont see next one anytime soon. At least not for two/three months at least. Though I might be proven wrong. (Though I hope not, I dont have enough FGems for this nonsense atm! )