All cats are different. You never know what her back ground may have been, what people before you exposed her to. A cat hissing is one that is letting you know that for right now, they need you to back off. It's not a sign that it's a bad or untrainable cat. She's new. She's in an unfamiliar environment. Try putting her in one room. Make that room her safe zone. Keep her there for a while, not just a few days. Let her know that this is her place of refuge and she will never be hurt there. When she feels comfy with that, let her investigate a little further. Cats like to have some boundaries, they like to know that they are in a safety zone.

I agree about the brushing. She's getting over stimulated and doesn't know how to deal with it. Just brush softly 3 times and wait. Try massage as well. use one hand, since she's a baby, and very gently massage up and down the spine area. It is super relaxing for them. They will feel safe and secure, and with time and patience, she should become a good companion!