Rainbows have all the 5* abilities plus taunt and weakness damage up.
Get them all enabled so they are available to girls as needed
only Kerria Alpinia Soapwort can use the Recovery boost (purslane will someday)
I use taunt a lot on counter types like Anemone and Evaders like Saffron, Lycoris pumpkin and to protect any 5* mixed in a team with 6*
I did Crit last because of the cost. Is good on Crit teams.
How you use them depends on what your facing, like for example weakness damage is only good in situations where you know what type you facing.
Always use attack up 1 turn on Whale on everybody except your deck team. I would start there boost them all to max.