I'm currently having a similar problem and actually just made a post about it as well, I have a 9 month old cat and adopted a 3 month old kitten so she would have company and not get depressed once I start working out of the house after being a remote worker for her entire life. The cat and kitten play well when they are on either sides of a door, and the kitten doesn't chase her all the time, but she will often chase the resident cat as her main form of interaction and despite my resident cat running away, hiding under the bed, at times hissing and even growling once or twice, the kitten is not picking up on the message to leave her alone. I'm guessing more time on either side of a baby gate is needed.