I agree that the cat is establishing dominance - and also expressing a sense that maybe it is a bit overwhelmed by the new dog. It sounds normal to me. My sister has a cat and she adopted a female greyhound - the most shy and gentle kind of animal you could hope to meet. But Lizzie, the cat, hissed and spat and put the dog in her place. To this day, the dog (and now, a second, male, greyhound) have their own room, to which the cat is a stranger. The dogs are free to come out into the rest of the house if they choose, but Lizzie has taught them their place, and they prefer the peace of their "dog room." I think that eventually, your two will work out a system that will allow them both to live in the same home.