When you add a song to the concert, the game seems to automatically pick whatever girl was center when you made the single. I don't know if you can change it, so I mostly had my "best" idol lead almost every single until I had a couple of successful songs. My first concert only pulled in maybe ¥300 000 profit, but a few months later I had some songs sell well enough that my second concert pulled in maybe ¥19 000 000 in profit. I'm still not quite sure how it all works.