Pilots: Level up enough so you can play with a balanced CV,a fighter-focused CV or a bomber-focused CV,and not put 9/10/11/12 Planes on 1 Pilot,it may take a while to bring up all those pilots (At least 6 Fighters and 6 Bomber's of whatever type you desire,maybe a Scout or 2 aswell) but it'll be worth it,course some people can't be arsed to do it,
R Slots: Mostly personal preference,some opt to use Gunner's for HH or even AA,some prefer having Rep's so they can repair from damage they sustained much quicker,or seaman to make their fighter's perform a little better (I Don't believe in them personally)
9th Sailor slot on PCV/CV6? People usually put engineer's there,so they can keep up with the battle line or get the hell outta dodge,some people put another rep/seaman or even a medic or restorer there (Though Medic's as a whole are pointless and a Restorer for a Nation that already has a 2+ Restore bonus on it's sailor's is quite silly)
Hope this help's.