@gingerw Hissing is probably not bad if it doesn't progress to the weird growling threats that cats do and then onto a real fight. You could get a dog cage so she has something bigger she could be safe in and they could still talk through the bars and they could have more time to adjust before they are physically together. My little nervous one has hissed ever since I brought her home at 12 weeks old I think out of fear. She was born feral and probably didn't have much to eat from her mom since she was found in a window well with her brother and mom in winter. Now she knows there will always be food to eat, but she is an alpha personality. She's the smallest one, half the size of the boy which might be intimidating to her, so she acts tough. He sometimes calls her bluff and swats back, and she'll back off. He's been very tolerant of her, and he does get out of her way a lot. He tries to play kitty chase with her, and she'll jump out of the way while hissing, but then she's not upset and pretends it didn't happen. She gets in trouble when she's serious about it for being the instigator and the squirt bottle comes out. She'll stop when she sees it, if not, she gets wet.

Even though they just did something bad, remember to praise them when they stop, and maybe give a treat when they calm down. She does sometimes do the more serious growling, and I had to put her in the bathroom by herself all night a few times. She really wants to be with me, so that is something that makes her think that is what she'll remember. The reward is when they get attention and playtime from you, so that is what you take away when kitties misbehave. I had to do this to stop biting which is normal when they are telling another cat to leave them alone, but I never want a cat to think that is OK to bite me, so isolation works, and make sure you don't cave in to the sad cries. They have to have time to think about why they were bad, and only be rewarded when they are calm. The best time to get them friendly together is at feeding time when they are rubbing up against you. Let that go on for a while while you are petting them close to each other and talk nice to them and feed them next to each other so they learn they both are being treated fairly. I put spoonfuls of canned food into their separate dishes, and add more as they finish so they are in that mode of asking for more, and the boy can't just take it all. Also play with both at the same time if you can keep them interested in toys. The other will watch and want to join the fun or maybe sneak in and steal a toy... my other girl kitty does that. You just have to be patient and give them lots of time, and they have to establish who is in charge which you won't be able to change. Horses do this too, and when I had to move mine to another boarding facility, he got kicked by the others over territory. He was moved around until he was happy with his paddock mate. It turns out that he is a lady's man and is only happy in the company of mares. If there were no mares, he would get along fine with the other boys, but with them there, he has something to get jealous and worry about.