Hissing and growling are ways of communicating, and don't always mean negative things. I have one girl, a rescued stray, who hisses and growls, and sometimes screams, at the other cats all the time. They pay no attention to it, it's just her way, they are all actually very attached to one another.
I tease her about her gutter mouth, saying she "swears like a sailor" as the saying goes. She just got in the habit of swearing all the time when she was raising a family all on her own.
At first the other cats were all quite shocked, and taken aback by her language. Then one time my Red Boy (the cat who loves all other cats) was crouched near her trying to make friends. She was dressing him down something fierce, hissing growling, spitting, it was really something. My boy just sat there staring at her in amazement.
Then, right in the middle of the harangue, she YAWNED! I laughed so hard I fell on the floor. No one ever took her seriously after that.
to the OP, please do let us know how things go, I'm sure with time she will adjust and even if she never stops swearing, your resident kitty will get used to it and win her over anyway.