Yeah, you can read about DMM bloom abilities (which we'll get eventually) on the DMM wiki that's covering the Japanese version of the game. They're well ahead of us on blooms. Ardisia's bloomed form isn't available on Nutaku yet, but I know what I'll be getting when she blooms. It's not a great match for my team, but I love her voice and personality, so she sticks around because I play subjective favorites and prefer a team of favorite waifus over the alternative of hoarding power and going for stats. It just depends on how you play the game.

Cactace is great -- she was rated pretty highly in the DMM forum's recent 6* tier list.

And, yes, rarity growth is essentially a mechanic that allows any lower-ranked character to grow into something like a 6*.

Sakura and Ivy have the same colors for their eyes -- I wonder if they're supposed to be related.