Re: Skills
So the short answer is — for any skill, there is an accumulation across ALL crew members and it is a general good practice to have all crew with a common skill set dependent on class of tank (ie., all with REPAIRS, all with CAMO, all with BIA, etc.) where those skills are none-crew-type specific (ie., the commander, gunner, etc all can have repair/camo/bia). The difference comes when you start really specializing with SKILLS and PERKS that are crew-specific. Example, Commander is the only one who can have Mentor, or the Gunner SnapShot.
So, I generally bulk up on the common skills first, then start specializing. QuickyBaby has a very nice set of videos that not only walk through this but he also provides recommendations on the order of skills for each tank type and crew member types.
Helpful?