Yup, I agree. No mom, first name is ok or a special nickname. She is not your child so there is nothing that you need to discuss with her mother. If she wishes to boss you around just tell her that it is between her and her ex and to leave her out of it. You and your husband can discuss the role you have with his daughter. Not you and the girls mother. If you have a good relationship with the mother that is another thing.
Do not force her to call you mom. When my fiance's daughter and I started to bond she wanted to call me mom, of course she was 7 at the time. I explained to her that it was fine when we are together but that I wasn't her mom and she had a great one already. Wanting to call me mom only lasted a while, now she calls me by my name. As far as discipline, well that will have to be discussed between you and your husband. I think it is ok to discipline the child as long as it's necessary, not just because you feel you can. You and your husband have to communicate on what he thinks is adequate discipline and boundaries. You don't need to deal with the child's mother. That's his job, unless of course you develop a positive and healthy relationship with her, then it's ok. I have a great relationship with my fiance's son's mother. We are good friends and discuss their son together. She actually calls me first before him to tell me things but I don't talk with his daughter's mother at all. I do not have any type of good feelings about her and so I don't feel the need to communicate with her at all. There have only been a few times that I have spoken with her and that was enough for me. Good luck, it's a tough job being a step parent but it's also very rewarding.
You could make it something cute and not as formal as your first name....maybe something like Kay-Kay??
oh, and i'd avoid dealing with her mom as much as is possible.
i agree this child should never call you mom. and i'd talk to your man and ask him to tell you what kind of a role he wants you to take.
I definitely dont think she should call you mom or anything like that, the first name is generally what kids call step parents. Having her call you "mom" is a BIG no no. You need to remember that she has a mom already. Have fun with her, play with her. I think you need to ask your husband how he feels about you disciplining her when the time comes. In my family, the step parent was never aloud to discipline, that was the actual parents job. I still had to respect my step father but he was not in charge of me. It's different in every family though.