Newly introduced hissing cats?

Make sure that they do not fight. That's the one very big important rule. The hissing should be natural and it helps them decide who's alpha and who's the lower one. One time something much like this happened to me, the nice cat was very aggressive and the grumpy cat was very passive. After a month or two of being bullied the grumpy cat fought back and became alpha. Now whenever another cat visits their turf they fight alongside each other. I guess maybe you need to allow them to fight but if it get's serious separate them. And I realize this contradicts my rule but its important. Cats aren't meant to be social, so it will take a couple of weeks at minimum. I hope I helped and that your feline friends become the best of friends :)

If they're both females, they may not get on. Females together and males together don't usually get on well with each other if brought home at different times. So because you've had your 8 month old first, she will class it as her territory. The other cat probably hissed because she was scared in the new environment and possibly because of her background. If she's from the shelter, she may have been bitten by other cats if she was a stray before being taken to the shelter. I would see how they get on over the next few days, and then make a decision from there. They might be able to work it out.

I have had to introduce many cats together. I put the new cat in a carrier in a bedroom and let the older cat seek it out on its own terms. This is also less stressful for your new baby. Soon the older cat will be lying next to the carrier. Then you can open the carrier door and let them be together. The new cat may run but now they have each others curiosity. That way the new one is removed from the others territory and of coarse curiosity is the key. Hope this helps you.

The hissing is normal, it will work out on its own, what you will want to do is make sure they don't attack each other. If they start attacking each other then you need to immediately separate them and start a slow introduction to each other.

just try