Yep, you can do that.
Excellent strategy if you have the roster space for it.
Yes. 3* only the new ones. treevil, finish grevle, sudri, shrubear, vollermork, keep in mind to level 1 each color, try to assemble tree 3* rainbow teams, every team with 1 healer.
Then go for the 4*. You have good ones. Don’t haste time and resources leveling S1 wothou costume.
The costume you can put the original a 1 in last ascemcion and work fully on the costume. If you want to put emblems you need to fully level the original. 3* at 50, 4* at 70. 5* at 80
Okay. So these are done/are getting done.
Then these
Then these… or these. Also depending what else I pull. 
Which kind of help you required !