New cat hostile to resident cat, please help!!

[haliborange0verdose]

Please can anyone help, I'm at the end of my tether and don't know what to do. Our new cat is so aggressive!
We have a nearly 2 year old ginger tom who we have had since he was 10 weeks old, and we adore him. For about the last six months, my two (teenage) kids have been going on about how he was lonely and wanted a friend, so about a month ago we gave in and adopted a female rescue cat. She's about 18 months old but had had a rough start in life - she got pregnant very young and was dumped with her kittens in the freezing weather. The rescue we got our original cat from, took her and the four kittens in, and they had dozens of applicants to adopt the kittens but none for her. I felt so sorry for her that I applied to adopt her. She came to us just over a month ago now.
Anyway, cut to the chase, she is VERY hostile to our resident cat. We did the whole slow introduction thing - kept her in a separate room for over a week, let them smell each other, etc. He is so docile, he's never done anything threatening, but whenever we let her out of the room she just hisses and growls at him. Even sometimes she hisses under the door. He just looks at her with this big dopey look on his face, and then trots off and hides under my bed. It seems like she either hates him, or she's terrified of him, but I don't know what to do to change things.
We've used a Feliway Friends plug in but that didn't help. A friend recommended I try a Pet Remedy plug in but that's not helped either.
If I could guarantee that in say four weeks or so, she'd definitely calm down, I'd just stick it out. But the thought of her being so hostile and aggressive to our poor lovely cat for the rest of her days, is too much!
I don't want to have to return her to the shelter - she had such a tough start that she deserves a loving home and we want to give her that. But she's making it so hard for us all!
Can anyone share any words of wisdom????

I’d separate them again and look at Jackson galaxy videos on YouTube.

A month in cat time is a very short period. If she can’t accept being a multi cat house she’ll have to be rehomed, your boy comes first.

I think your kids opinion on what cats want and don’t want is inadmissible though, teenagers don’t consider the long term problems introducing two cats can bring.

I moved in with a flatmate once and our cats loathed each other. My vet told us to to wipe some tuna on their fur and put them in a small room. The theory was they'd lick it off and learn to love each other.
Half an hour later, the lav smelt like tuna and I had two cats glowering at each other in opposite corners. They never learnt to get on.
The smell took a while to get out too.

Happened again when I adopted two cats together and their relationship changed. Apparently it happens quite a lot. We had to return one.

Have you tried normal feliway? Mine don't respond to Feliway Friends but Feliway is much better.

Can you stage your intros so they aren't in the whole house together? Or get some time apart?

We did slow intros but then did room swapping, new cat had upstairs only so old cat always had their own space etc etc.

We are about 6 weeks in and having stand offs on the stairs and having to make efforts that each cat gets lap time without a war happening so no-one is just sulking in a cupboard but I think we are getting there.

Having said that old cat is currently monitoring the stairs for a cat that isn't there

I'm sorry to say this as I'm sure it's not what you want to hear, but I think there are some cats that will just never adjust to a new cat in the home. Our cat was a rescue cat too who had had a bad start in life - she never really recovered from that trauma. I suspect she was taken away from her mum far too young and was not socialised as a young kitten. We always planned to get another cat but were not able to, as her aggression towards other cats was so bad, she couldn't even see one outside without violently trying to attack the window. It became redirected aggression and she also attacked us, as she thought we were the cat. We tried everything, even medication, but she never really calmed down and was like this until she passed away in March.

It may be that if you want both cats, you will have to keep them completely separately.

Thanks for the replies, I shall take a look at those videos on youtube. They are still separated, she is in our son's bedroom most of the time and really only comes out if the other cat is asleep somewhere.
We've tried doing "supervised" meet and greet sessions today - keeping them apart but looking at each other. She hasn't hissed but is still growling and I can tell by her demeanour that she's very nervous.
We really don't want to have to rehome her because we do love her, but obviously our first cat has to take priority. If she remains unhappy and he also becomes unhappy then it's no good for either of them! We really want to make it work!