As a f2p player you get 3 gold units Katie, Iris, and Cloris...at what point would you start using gold spirits to level them? Or other units? When you use them should you do 2 fodder 2 spirit, or some other combination?

I'm currently at 50cc60 on Katie and Iris on both my accounts, but I'm unsure about when to start using the spirits just for leveling...I have like 50-60 on each account and no additional gold units to bother using them on. (well second Iris on both accounts but one already has Marr as a third healer, and I AM leveling the second Iris on the other.)

Thanks for your time!