Firstly, if you love this man and want to marry him and you know for sure, that's your decision, regardless of how many wives he's had or how old he is!
Does the 8yo come to your house, his house? If it's your house = your rules full stop. Just like you said, she reminds you of you at that age, but you had to respect your parents and that's exactly how it should be.
If it's your partners house, then he should be given the respect he deserves. I understand she may be hurting from the break-up between your partner and her mother but that's no excuse for that behaviour. Children from a very early age, work out what they can get away with and what they can't and it's up to us to set the boundaries.
I believe you need to sit down with your partner and come up with some house rules she needs to follow, i think it's also important to explain to your partner that he needs to be a little more assertive with her. It may end up yelling and screaming with her for a while but in the end hopefully, she will start to understand. All the best!
Two thoughts, mzmandy.
You are too young to be with a man who is as old as he must be. He's immature, to pick a 23 year old as his third wife. There is virtually no chance your relationship will work out - he's already got two strikes, and he's picking a woman who can't stand to be around his child.
If you insist on marrying him despite all this reality, and you are TRULY willing to do whatever it takes, welcome this child and stop fighting it. She is who she is, it's not like she's a knife murderer, she's one of those girls who has to be the center all the time and is uncomfortable if she doesn't have control. You can either agree to accept that cheerfully, or fight it, either way, it won't change anything at all. Well actually if you accept her cheerfully you stand about a 5 percent chance of succeeding in your marriage. Which is some odds, after all.
What does your family think about this? Are they happy you are marrying this old guy with two previous failed marriages and two daughters?