Well i have a sub crew but it's just... plain (no lateclassing sheningans).

But from what i learned AFTER i made my sub crew:
Planesman and Sonarman can be late classed to maximize repair and restore growth (classing them from support lowers the growth for "irrelevant" stats) so you can SD cap easier and sooner. It also lowers the # of crew whitch while lowers the ability of the sailor also reduces his size and you will probably have to ghost some sailors anyway.
I'm not going to talk about how to late class them because frankly i don't know and don't
want to provide you with false info.

KM subs are as good as IJN but specialize in killing other subs and smaller ships.
IJN subs are capital ship killers. 

SS1 - 6 front torps
SS2 - 6 front torps 2 back torps
SS3 - 4 front torps 2 back torps 
SS4 - 8 front torps
SS5 - 6 front torps + Can carry Torpedo bombers.

I don't think seamen are important better put another engy or a repairer instead. 

Also, i saw that the Junsen has 20/20 in the guns. Is this 2 sailor operating each gun or 1 sailor operating both guns?

I think thee is only one slot for a gunner so it'd e 1 sailor for 2 guns.
In general subs with guns only need one gunner i think.

But seriously speaking noone uses guns on subs aside of the odd KM SS4 (those machineguns tear things up like noones buisness) 

i have all ijn ss, but ijn ss4 hei is the best over level 100.  No doubt

Realize on the Junsen,  engies in gun slots provide no additional speed.  Only put them there to class up the sailors and leave the recruits as low as possible.  That is where I put my Seaman to try to get additional speed.  He doesn't add much, though.  You can also put a repairman there and he will be effective.

Bobys,

    All of this information is in my SS Guide.  I just reorganized it so you could better see the info you want. 

    For the delay of classing, here was a post by Haslam on the third page of the guide.  Its the most comprehensive info I have seen about it.