From what i have seen people pull the rares come at 30-40 scrolls and super rares at 120, might be wrong but it seems consistant.

Now regarding the ninjas the only 2 star that is actually good even like that is GNW Tenten and I alreadu have her.
You do have other good ninjas like Mu, Rasa and GNW Hinata, but 2 star tanks aren't a good option.

But feel free to do it if you really want them now.