I totally agree with Wannabe Catlady. A kitten does not deserve to be "returned" after less than 24 hours of being introduced to a new cat. There is NO way I would ever take one of my cats to a shelter - if they do home meet and greets like some rescue groups with fosters do, that would be ideal, but I wouldn't take a cat to a shelter (especially if she's never been around other cats - she will be completely stressed out and any sort of reaction you get from her would not be reliable). As Wannabe mentioned, it's stressful for both kitties, and they need both need time to acclimate. Give the new kitty a little time to adjust to being in a new home, then try to introduce him to your kitten. Hissing is not a big deal at all, and is perfectly normal - that's just him trying to establish territory (again, he's just stressed beign in a new place). When he does that, you just need to back up and give him his space. Let him approach you on his terms, not the reverse.