Thank you all for the comments. We put the two older cats (Maya and Kai) on a low dose of Prozac for the past 5 weeks and the situation is doing much better! Maya (gray) will still chase the kitten from time to time but the kitten now growls back and then hides. Kai (black) is so much more confident now, she was always very nervous, and even tries to play with the kitten sometimes, but the kitten is too rough so it only lasts for a few seconds before Kai (black) hisses at the kitten. We think we're going to keep Kai (black) on prozac indefenitely because it has helped her calm down and not be so vigilant/nervous all the time. We're going to keep Maya (gray) on prozac until the kitten gets to about 1 year old or starts to calm down. The kitten is 5 months old and about 6-7lbs now, and just overall less crazy, so I think a combo of the meds and the kitten maturing has improved things. Definitely the hardest cat introduction I've ever experienced!