I am coming from the other end, so not been there done that but I am a mom who does have a good relationship with the soontobestepmom, as I call her. (Dd's dad and her are getting married 7/3/10.)
Your partner's ex sounds like someone who you may never have a good relationship with, I hate to say. She sounds pretty frightening. Did you and her have a tumultous relationship (that involved your participation in conflict other than just being a victim of her harassment) at anytime?
If I were you I would be looking into restraining orders from what you describe. Youch!
That aside...If you do want a good relationship, here is what I have to say, from the mom end. Let the bio-parents hash out the details such as child support if that is a factor, child-rearing, discipline, etc. Maybe not forever, but let everyone kind of find their bearing and just kinda transition into the changes, give everyone time, including yourself. Be open and friendly, but if you try to be her new BFF that might be weird too.
Be a support person for your step-child and do your best to, while the child is transitioning to the new marriage, allow consistent dad/child only time. This will make it easier for you in the longrun, and the child will take to you better...even if you have an already established relationship with the child...marriage can feel like a big deal so this small reassurance that he/she still has her daddy is crucial.
I don't know how sane your partner's ex is....but remember that even a sane, thoughtful mom will get a little mama-bear or defensive when she feels like her child might be slighted. I know that in the few times my co-parenting team has had issues is because my daughter was upset about something, and I felt like I needed to be her advocate. It did create some conflict, but I told my daughter's dad that I will always fight for my daughter if I feel like there is something going on that is unfair to her. He gets it and works really hard to make things positive for our dd, so does the soontobestepmom. Also, do your best to love that child like your own....I am not in your experience but I have been a stepchild; kids can totally sense it when they are resented, when other kid's interests take precedence over their own or when someone feels put out by their presence.