There's this gacha simulator, it's for the JP version so has unreleased girls but you can still test your luck.
http://flowergacha.web.fc2.com/index.html

That link fills me with no hope. On that subject, does anyone know what % changes are in effect for the limited gacha system? Does the Rainbow 6* raise from 0.5% to 1% for the 3 girls that are on? Or does it stay at 0.5% but only selects from those girls?

The chance for the rarity itself stays, just the chance that if you get the rarity it is very likely to be the promoted character.