I don't think the leader needs to be the best singer of the group, but maybe it shouldn't be the worst. And if Reina would stand out too much, just make Haruna leader. She has a dignified look about her. (Okay, usually).
Ugh, I'm completely against Haruna getting leader next, Mizuki is more than likely going to get it next (due to her being a former egg, and basically the one with the most experience.), despite the age difference between her and Haruna. There's just something about Haruna that I just don't like :/ Who knows, if they do sub-leaders after the rokkies leave, she might get it.