These additions make me wonder if they were chosen for very specific reasons to make them more well-rounded and improve various aspects to their character. People seem very shocked about who was put where, but it clicks when I think of it like this.
-Nanami is very child-like but lady-like. CG was PERFECT for her, but besides Tsubaki, which could be argued, she needed a more "mature" image to fit the other groups. Why not throw her into the most mature group to improve how she appeared. J=J always appears kind of tomboyish to me. With her joining J=J, it gives her much to improve on. Plus, she could sax solo early J=J songs.
-Ruru's vocals are quite good, but she never has been able to properly use them to her advantage. Even in Angerme and MM, she would have issues standing out with her stellar vocals. Sayuki is arguably one of the best vocals in H!P. Add her to improve her vocals.
-Chisaki is very shy. Why not put her in a group where she will have the most events and have to socialize with more people on a daily basis? Plus, by having a skilled dancer in the group, she will improve her dancing as well as singing. MM is in need of another front girl, and Chisaki was being pushed as the front-girl of CM.
To me Musubu's and Ayano's personalities are a given why they were chosen for Angerme. It is also the most well-rounded group. The group will help showcase their personalities in a group FULL of crazy personalities, while still having to work at it. They have solid basis in dance or vocals, but both can use some fine tuning. Energy is what created what Angerme is today. The group thrives off of it.