Dealing with NPD and having another relationship is very tough for me. I am one of those people that have a very hard time keeping my life in compartments. I think this helped and hindered my relationship with XSO.
Helped because he was looking to be KISA and I was so broken down from dealing with crisis after crisis with X that I needed someone to emotionally support me. He was able to cut the NPD drama for me down to what needed to be dealt with. I needed that. I often took his advice on things relating to NPD and kids. We made a good team.
The hindrance : we made a good team. When the drama started to wain as the kids got older and of age, and NPD and I had less and less contact. XSO felt less and less like he mattered in my life. As I grew in strength I needed him less, and he needed to be needed more.
If your dealing with NPD, and have underage kids, the drama continues (usually). The new partner sees/hears it, may even see/hear how you deal with it. I see NPD drama as being no different than having a fight with your best friend and venting to the NP. Just don't make it the basis for the relationship or let the NPD drama into your new relationship. Venting is ok, letting NP support you in what you need to do is OK. But the focus needs to be on you and New partner...I still wonder if all the drama with NPDX in the early years was the glue that held XSO and I together?
So learn from my mistake.