I played Nellie in a semi-professional production of the show, and the role was LOTS of fun. She's the madam of The Red Rat (the brothel where Lucy and the girls work). She has two featured scenes that require good comedic timing, sexual innundeno, and a Cockney accent. (On Broadway, Nellie was called Guinevere and doubled as Lady Beaconsfield. The licensed version of the show is different.)
I performed (as Nellie) with the girls in "Bring On the Men" and "Dangerous Game"; played several ensemble roles in "Facade," "Murder, Murder" and Emma's engagement party; and sang the many off-stage vocals.
In addition to the specified leads, our ensemble had about 16 (4 on each voice part - SATB). For the vocal demands of the show, I wouldn't suggest a smaller cast - unless you have very capable singers who can sing 4-8 part harmonies with a solid, full choral sound.
Hopefully this answers some of your questions. If you have any more, just let me know.