Delay classing (as your talking about) is generally only for specific types of gunners (IE IJN AA gunners, UK AA gunners and for those bold enough USN AA). Other sailors can be delayed to create higher secondary stats however these are usually offset by massive vets on the sailors.

It would be best to search through gunner classing topics for specific nations as different nations have different classes available and different paths. If you can not find the specific information you need at that point just start a new thread in that nations section.

In regards to boosting a sailor, the boost is applied to both Growth and Ability meaning that the same bonus you recieve at boosting a lvl one is recieved if you boost at level 120. You will get more benefit from the boost at lower levels as many sailors hit a cap in later levels anyways (in particular gunners).

So as long as have good growth, I keep recruits up to date, and class at level my sailors will turn out ok?

What about vetting and experting? Is it best done as early as possible? or does it just need to be done?

tl;dr is there anyway i can render my sailors useless?