Responses
Niang Niang should be for all consorts, it's just a way to politely greet them. Fei- Should be the title. There is Empress, consort, concubines, etc. I believe there is the Royal Concubines titles too. I think there is 1 Empress and 2 Royal Concubines for the emperor to crown.
You mean huanghou,guifei & fei?
Yes. I only know them in Vietnamese
If im not wrong then niangniang is a way to call consorts & empress or a way to say her highness but not mother cause in the imperial harem you called your consort mother "mufei" and empress mother "muhou"