I can't see on how well the anime will work out since it has a huge cast. And why Nina of all people?

Neither said:

I can't see on how well the anime will work out since it has a huge cast. And why Nina of all people?

Popularity, probably. Well with Nina that makes two CD debuts before the show, but I think she should be in the cute category.

For what I have seen, the next five CD debuts will be Yuki, Momoka, Fumiko, Nina, and a another one from cute category.

Neither said:

I can't see on how well the anime will work out since it has a huge cast. And why Nina of all people?

The main Idolmaster anime had 13 core idols. Cinderella Girls is focusing on a core of 10. Should not be an issue.