My sister died 2.5 months ago and I inherited her cat - Buster - he is a neutered 18 month old male. I already have a cat - Itsy (neutered female) who I have had since a kitten and is now 8.
Initially we introduced them gradually, a few hours a day but Itsy always hissed and spat at Buster and often left the house for long periods (which was unusual for her).
After about 7 weeks I spoke to the vet about this problem as it was getting harder and harder to get Itsy back in the house - it was as though she was scared to come in.
I got Felaway - happy hormone plug in from vets and Itsy calmed down and stared coming back in the house when I called her - but always went into the spare room where we put her food and she slept - as she was sleeping in there all the time we didn't feel it was fair to let Buster in. He seems very playful towards her and there has been a number of chases which do lead to Itsy hissing and spitting a lot and then running off.
We are now almost keeping both cats separated as we don't want to scare Itsy off - the past week or so Itsy it is getting harder to get Itsy into the house - I don't know if there has been an incident but again she seems to be very scared of him.
Should I let them fight so they can establish who is top cat? I am very protective over Itsy and she does seem scared of Buster - Buster has settled into his new home very well but it feel like he is pushing Itsy out. Advice please as I am very stressed out!

I would keep them separated when you are not around and let them be together when you are so if the fighting gets too bad you can break it up. As for her being scared and unning off, I'd keep he in the house until you figure out what's wrong, better safe than sorry.

Itsy is angry... do as charlotte says and just don't leave them alone until you know no one is going to get hurt... They will learn to live together though they may never be playmates. I believe it is better to have 2 cats.

If I only had one cat, he'd be so bored and lonely when I go to work and go out to do things, they has toys and stuff, and I dote on them so much, but it's no substiture for another animal there with them.

I recently also added but it is so much better the cats will lay near each other sometimes now they still will hiss and swinanother cat and it was very tense and difficult for 6 months there is still some tension now 8 months later g at each other if they feel they are crossing the line with territory but it does get better, I would as the other person said do not let Itsy out just in case one of these times she doesn't come back and do keep the cats apart when you are gone and when you are home keep a distant eye on them so if it gets bad but give them some space to sort out what ever their issues may be. It will get better also because of the stressful situation that brought this other cat to you the cats could be feeling some tension from you so as hard as it is try to smile and relax around them when you are all together and best of luck to you