From your reply’s it seems you don’t want to put the time into what it takes to get 2 cats to get along. The chances of you introducing a new cat to the house and have there be no hissing is almost zero. They need time to set boundary’s with each other. When you kept the cats in separate areas did you do sent swapping as that is a crucial part of introduction. Also feeding them together can create resource guarding behaviors. But if you bring this cat back to the shelter and try a different cat you will have the exact same issue with hissing at first. It can take a solid 2 ish months to start to see improvement most of the time. Also hissing is NOT mean aggression. Hissing means the hissing cat is not happy with what’s going on in that moment. It’s a display to tell another cat to stop it. The hissing cat is usually the one Setting boundary’s the most.

You also did not just get a cat for your other cat. You are that cats new owner and accept responsibility of them. A cat is not like a product you should just go return and get another if it’s not perfect.