I'm sorry to hear that Paizly seems to be so stressed out. Have you ever tried using Feliway spray or diffuser? This may help a lot because it has a calming effect. She's also probably hissing at everyone because she's trying to let you know that she's upset with this new arrival. In my experience this has only lasted 3-5 days but I know that some cats take much longer to accept another cat. My RB Sunny would just try to ignore the newbie and he wanted me to baby him even more. All cats are different so please try not to stress out too much because your animals will pick up this stress from you and they'll also become stressed. I'm sure that things will eventually work out. Good luck.