We got Gizmo when Hunny was just about a year old. Hunny had just recently been switched over to Cat Chow herself, but as with you, there would really have been no way to keep their food separate so we gave both of them Kitten Chow.
By the time Gizmo was a happy & healthy 6 month old cat, while during a routine exam, I asked the vet if it would be OK for her to be on Cat Chow, since 6 months on Kitten Chow had caused Hunny to gain quite a bit of weight and I was worried for HER health.
Because both of my cats are seen regularly and we never give them anything but a normal cat diet, they are amazingly healthy kitties. Because Gizmo was growing so well, the vet agreed that she could be switched over to Cat Chow at 6 months.
It all depends on their overall health, since as previously mentioned, Kitten Chow has essential nutrients necessary for proper growth. On the down side, it has too much of the good stuff for the adult cat that no longer needs it.
No harm in Isis getting Kitten Chow, as long as you keep an eye on her for signs that she might be gaining too much. If she's spayed this could also have a bearing.
If if becomes a concern much more before 6 mo, or at that 6 mo point if Mao seems to be growing well, healthy & happy, see if the vet would agree that Mao can have Cat Chow.
Good Luck!
