Not sure what you mean by Xiamen airlines plane but KA never utilizes Xiamen airlines plane on any of KA's flight. When there is AOG or last minute rotation, CX plane might be flying but usually that is only to TPE or KHH.

KA currently only has 6 airplanes with F and these six planes are flying between PEK and PVG only. The chance to get on F is there but since you are not flying until October, it is very hard to check it now.

Checking April's schedule, there will be 3 flights each day with F
KA804 1055-1330
KA808 1655-1930
KA894 1900-2125

Since your schedule seems to be flexible and you are arriving Hong Kong early at 7 am (before all PVG flights depart), I would suggest go to the transfer counter upon arrival and ask for which flight with F and ask them to put you on that flight, that's if you are not already on those flights. With your ticket being F, I don't think there will be any problem.