My older cat ran into the room to where Im keeping my new kitten, and she hissed and growled, what should I do?

Hi, I have a cat who is a 7 year old small female. She recently ran away and since she is an indoor cat we thought she'd never come back so we started to look at new cats. She came back 17 days later, but not before I fell in love with a little 5 month old female kitten. I was intending to keep the kitten away... show more Hi, I have a cat who is a 7 year old small female. She recently ran away and since she is an indoor cat we thought she'd never come back so we started to look at new cats. She came back 17 days later, but not before I fell in love with a little 5 month old female kitten. I was intending to keep the kitten away for at least two weeks from my older cat so they can get used to the smell of each other, but my older cat ran in the room to me, and when she saw the new kitten she hissed and growled, I tried to calm her down and show her that everything was okay, but she was not liking the kitten one bit and scared the poor thing. I'm kind of angry at myself for being so careless and letting my other cat in like that. Is this a bad sign of the future? Am I way over thinking the incident? (I remember that when we got my first cat, she hissed at one or two cats next to her at the shelter.) I am not going to give either away, but I don't want to have to change my house rules so that they're always separated and that people have to be careful. Could it possibly get better? Any details on the matter are much appreciated!
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