Obedience Training Advice for a 6 Month Old Puppy?

I have rehomed my American Bulldog and she is such a good girl. Although her previous home was a very chaotic environment with unstable people. She's basically house trained apart from the odd wee at night, sits down before crossing the road and knows all her basic commands. She is quite nibbly though and bites (not overly hard) and argues with me.

I've had an American bull dog before, but from a very small puppy so it was a bit easier in asserting myself as the leader and I am used to all the talking they do so am prepared for the odd talk back. I'm pretty sure she's trying to assert dominance with me though. Does anyone have any advice on dominance training for older pups that has worked for you?

I think because of her previous circumstances I have been so concentrated on settling her in and making her feel comfortable she's kind of took charge in her own mind. Also I am aware it will take a few more months to fully settle her into our home. She is very obedient and loving when she isn't having these moments and has been an extremely quick learner so I'm positive she will respond great to any training tips I get. Thanks in advance.