My parents were divorced when I was 7, and both were remarried within 2 years. I have spent the majority of my life HATING my stepfather because he came in and wanted to set all the rules and be "the parent" and control everything. It became me vs. him, and it came to the point where we couldn't even stand to be in the same room as each other. It pulled my mom and stepdad apart because she was always in the middle, often siding with me and my sisters. They hate each other now.
My stepmom was loving, but distant. We knew she didn't really like us, but she was wise and patient enough to let my father parent us when we were there. When there was a problem, they discussed it, and then pulled me into the conversation. When I warranted a punishment, it was from my father. She was stoic and stable, and I knew she was gritting her teeth half the time, but we always respected the fact that they loved each other, and she let my father take the lead with us. It was incredibly disrupting to have the divorce, but the remarriages were 100 times harder.
Your job as a stepparent is to be there as a support to your spouse and a mentor to your stepchild. (This does not mean letting them get away with stuff, or encouraging bad behavior). It is the BIOLOGICAL parents responsibility to discipline, and both your fiance's and his ex wife's responsibility to decide what the rules are and enforce them (or not).
If you look at this situation, and you don't like how he parents, you need to step back and look at HIM.... you are going to be raising his daughter, and any future children you might have with this man. Parenting is HARD! It is HARD when they are your biological children that you love to death! It is HARD when you and your spouse totally agree on parenting styles and discipline techniques! It is impossible to parent, or to be in a marriage, where you are resentful of his parenting, you are trying to do more and he is undermining you, and the dynamic becomes you vs. the stepdaughter with him in the middle.
DO NOT MARRY HIM UNTIL THIS IS WORKED OUT. It will be a nightmare for the child and for your relationship. Good luck. Make an appointment with a therapist. It will be the best money you have ever spent.