I think you guys summed it up nicely.
Pro: vocal variety. Wasamin and Komori can sing and they lower pitched voices which could make the songs more interesting. On the other hand, it would make the songs less cutesy, maybe less 'WH'?
Con: The characters. Wasamin seems pretty shy around her senpai so she might not be as high energy and 'out there' as the other girls. Komori may be too tall to fit in with the overall picture:
Imagine Komori in that line-up. She would really stand out.
I don't think that the number of girls would form any sort of problem though.