Any cat experts??? Introducing baby kitten to only cat.

Bailey • Ava Madeline🧸 07/23/2020🌈 IVF baby

I’m big on bottle feeding and fostering and I got my first cat almost a year ago as a singleton and he has never been around only cats it’s been only him in here. I recently fostered a sweet 12 day old calico and I really want to keep her! The only problem is my 11 month old male cat I walked by with her and showed her to him and he started hissing and got all defensive and she’s so small it’s not like she’s a bigger cat. I just want to keep her and have them as friends. Any advice? Should I just let her walk around instead of being isolated? He is neutered so I don’t know what the deal is with him. He loves my dogs since he grew up with them but I want to have 2 cats and now I know it might not be possible😢

This is my older cats face when I was bottle feeding her hearing her cry😐 evil right? Lol

And this is the sweet little angel I really want to keep!