new cat aggressive towards old?!

That is just what cats do. Some cats take a long time to get to know each other and some adjust right away. If they were going to hurt each other they would have by now. One has to establish alpha-ness over the other. Hissing will probably continue for awhile. I would leave doors open and allow both cats escape routes to get to if need be.

Otherwise I think you are doing fine. Just be patient.

I've been going through the same thing for almost 4 months now. The newcomer, Tonya, stays in the guest room. Some of the others visit her from time to time, usually when I'm in the room. Right now, however, I'm at the other end of the house, the gate is open, and it is quiet, amazingly. Tonya is probably under the bed.

Tonya is afraid of my other cats and hisses and growls at them. If I take her out of her room, she runs back as soon as she can.

Cat people I know (and who know Tonya) suggested I shut her out of her room, but put a shelter of some kind by the door for her to escape to. I got a shelter, a padded cloth pyramid, and it's been in her room about three weeks, but, so far, I've never seen her in it. I haven't tried yet closing the door with the pyramid as her option of escape.

But maybe the day is today.

Anyway, your new cat is hissing and growling at your other cat because she is scared. These introductions can take a long time for some cats. Just be patient. It will work out in time. It will even work out at my house with my Tonya. Eventually.


Robin

Sounds like it is going really well!

At my house yesterday, I didn't lock Tonya out of her room, but she did venture into the hall and bathroom next to her room - when everyone else was napping. But also, my male cats, Claudio and Harlow, spent time in her room, with and without me, mostly napping, and there were no fireworks. They miss that room, as they both used it for afternoon sleeping before I brought Tonya home. And there is that special place, under the bed, where there is a cushy velvet mat in front of a heat vent, that nearly every cat I've had living in this house has coveted. Claudio got use of it for a little while yesterday. It's Tonya's safest place. She peered under that bed and looked at him the whole time he was there. Eventually it must have disturbed him, because he moved to the top of the bed.

Most people here would say that 2 or even 3 litter boxes are needed for 2 cats. I use 4 litter boxes for 5 cats - but one is in Tonya's room (and one of my other cats uses it regularly, as she jumps over the baby gate on the doorway of that room, any time she wants). Eventually I will try to eliminate Tonya's box. But I will wait until she is freely and comfortably occupying the whole house.

It really depends on the cats. But, if you want to go to one litter box, I think waiting until they are fully comfortable with each other would be good. You don't want one to be scared away by the other from the litter box.

Robin

Yes, little by little, progress is being made at my house. And that makes me happy

A black cat! Isn't that great??!!!

Robin

Now part of that is just cat "communication"/what happens when introducing cats. But I have noticed that working with so many cats and owning some that female cats tend to be more dominant. Or female cats tend to be more "quarrelsome" then males.

There is very little you can do except let the cats work it out themselves in my experience. But you could try giving the female cats treats when in the presence of the male.

I do not see anything wrong with the female cat hissing at the male. As long as she is not attaching him its fine, IMO. You can praise (treat, toy, or verbal) her for being in his presents.

Also how long where the female and male cat separated before introduction? Did you "swap scents" prior to physical introduction? How long have they been in "physical" contact?

Lastly most cats like treats, just finding there favorite treat can be very hard. So I would just experiment until I found a treat she could not resist. Like one my cats loves Pounce semi moist cat treats and one loves Whiskers Temptations.