Advice needed concerning new cat

[Jadorechats]

Hello all,

Earlier this year, I posted about a cat in my neighbourhood. I have been feeding her all along. This week I decided to give her a home. She came today for some food, and I put her in the carrier and we went to the vet. Happily, she was given a clean bill of health...no feline AIDS, no lukemia, etc. She did have some fleas, and an open wound on her side, and both were treated. However, now that she is home, I am having some issues.
I have an 8 year old male cat, who has had the run of the house. Right now, I have the new girl 'segregated' in a bathroom for fear of fighting. They are both very calm. I am not trying to rush things, but I have let them see each other. She is calm and inquisitive, he is upset, hissing and howling. I know this is to be expected. I ordered a book concerning multi-cat households, but I didn't expect to take her in so quickly, and no telling when it will arrive.
I am completely upset not knowing really what to do. I went out to the store, but they did not have any Feliway (I plan to look tomorrow).
Another issue, this new cat has always been very friendly, allowing me to pet her, even running up to me wanting to be petted... but today, after taking her in, she has started hissing at me. She will meow as usual, and then follow with a hiss. This is when I am in the bathroom with her, and perhaps it is because I am standing and look imposing to her? I talk in a calm voice, and get down on the floor at her level, and she will allow me to pet her, and rub up against me, but when I stand to leave, she will hiss again. I am not quite sure what to do. If my local bookshop had a book on introducing cats, I would have bought it, but I had to order one. I have heard that every situation is different, and there is really no way to tell how long it will take for cats to get used to one another. The male seems to have no interest whatsoever. He does not sit at the bathroom door, nor does she meow wanting to get out of the bathroom. Every time I enter, she is quietly curled up on the countertop.
Also, I don't want my boy to feel neglected, or that he has been 'replaced'. I've been showering him with affection and treats, but he does seem a bit different. They are mysterious creatures, but I am feeling very bad. Any advice will be greatly appreciated.
Thank you

[fjm]

It sounds to me as if you have made a good start - you may find the very helpful Catchat information page on reconciling cats to each other: https://www.catchat.org/index.php/new-cat-introductions useful. It is still very early days - one cat has to get used to being confined, and the other to having an intruder in his home territory, and both will take time to adjust. If she is resting in the open and happy to greet you when you are not too big and looming, and he is not hiding away or obsessively guarding the bathroom door, I think you can feel hopeful that given time and patience they will adapt to the new situation.

[Kay]

I too think it could be a lot worse - you have to remember that the newbie has always known she could run away when anything bothered her outside, but she is now in an enclosed space with nowhere to go, so it is only natural she feels a bit threatened now and again. The fact that she still interacts with you when you are at her level shows she is not overwhelmed by her new experiences.

It is fortunate your incumbent is a boy, as female cats tend, in my experience, to be much less willing to accept a new companion, especially another female. I think you need to set your sights fairly low to start with - if they do not physically fight, you can safely ignore any growling and hissing - aim to allow each of them a safe place where the other is excluded, while they adjust to each other.

[Jadorechats]

Thank you all for your advice and support!
This morning, the roles were reversed, as she was hissing, and he ran away... however, it was because she ran out of the bathroom while I was going in.
I am happy to say that the Feliway has been ordered, and will be here tomorrow.
I am going to study the new cat introduction tips and start putting those into practice...
I just pray that with time, things will work out.
I really appreciate this forum, and especially everyone's kind support.
Hopefully I'll have a happy update in time!