Will I have to find a new home for my new kitten?

Is he biting her under her neck or up between the shoulder blade area? If he is biting in the shoulder area he may just be trying to pick her up. I know he is a male and shouldn't be acting like a mother but he just might be doing that. It's not that uncommon for him to try to do that and that may also be why the kitten is acting like she is afraid. What you need to remember is your male cat could very easily hurt her in a minute if he wanted to so I don't really think he is going to. I would probably separate them when you go to work until your little girl gets bigger. Also the kitten may still have the scent of its mother and sisters/brothers on her, that would make your cat a bit confused, it's a new smell to him. I really wouldn't worry about having to get rid of your new kitten. Another sign that your cat is being territorial is if he is spraying, marking his turf. He will probably end up doing this when the female is older and in heat but if he does it now it is basically to make sure the new kitten knows who's boss. As for the hissing, it's totally normal and will probably continue on and off forever, mine still do it to each other now and then.