If your are going for CV line you need to start from the following bases:
Will Need: 1 BO (Pot 14 or 15) , 5 to 6 Fighter pilots (Stat 12 en Fighter) 2 or 3 Dive Bombers (stat 12 in Bomber). Thats the base crew.

Additional sailors could be: 1 -2 seaman (Stat 12 FP / 11 DB is almost impossible, but seems to be the best) and 2 AA (stat 12 rel) that can be used as AA or with HH against SS.

IF you clased a Gunner sailor, engine, repair, etc as special force only to have the pilots for your CV, you made a huge mistake. Those pilots will be useless in the future.

Search more information, or talk with high level CV´s for advice.

I hope this help you. 

To use armaments, you will need to classify one or more as
'Armament Sailor' at level 12. Even though they may have the ABILITY (the + beside their stat) they will need the specialization (whether Support for engineering/repair/restore; Special Force for pilots/marines/paratroops; or Arm Sailor for gunnery).

Hope this makes sense and is of assisance to you.

it probably can't use guns 5" above

Actually if you have a look at the components list here  on NF.com - even the most basic guns are classified as operated by sailor or armaments.  I think the ones you were able to use were the sailor ones and the level 21s fall under armaments.

Well we've known for years that the info in game and on websites etc do not always match.  Perhaps the simplest way to look at it is, you are trying to use sailors that aren't really specialized in that field so any gun you can use is a bonus.