Oh no I think sedation is what's making him hiss. He's probably very disoriented and confused and nothing is what it seems, and being slightly drugged increases that effect. I'm sure he's just very scared. Poor Buddy and poor you - how stressed you must be . But I think all will be ok once the drugs wear off. And here's a link on how to introduce a new cat to existing ones . I hope this is helpful. Good luck - I'm sure it will work out. Buddy is very cute

http://www.ddfl.org/behavior/catintro.htm