Having a skeleton BO is a must, potential doesnt help sub vision range and it will only slow you down to run a full BO.

Torpedo ability caps fairly quickly, so with enough vets/exps you can trim off your recruits without it having a negative impact on torpedo reload. Its not essential at low levels but it is advisible at higher levels.

Most SS guns are just a novelty so having a gunner onboard isnt worthwhile, you can save weight/gain speed with gun slots empty or put a Rep/Restore sailor in them for boosted support abilities.

As for supports, do not skin them down. Sonar, Planesman and Engine are all extremely important abilities for SSs.

Its true i run 110 torpers and they have about 109 vets and 500 experts, havent used recruits since SS3

I assume, if you're going to skip sailors though, you then-want a torpedoman emphasizing true torpedo-ability?  I.E., not a repairmen levelled as a torper for a little extra repair (have seen some suggest this)?

Wouldnt "skinning them down" reduce the ability to produce vet's expert's, and if not at what ratio is it advisable to "skin" them.

here we go once again...
for a sub, #1 thing is speed, so:
skeleton sub BO is a must = 0 rookies, 0 experts, 0 vets
torpers should be trimmed to just have torpedo reload cap.
sonarman, planesman advisable to have not only high potential base, but also high repair and restore bases, because they will be fully crewed.
engies and reps you can use from your BB crew.

now why you need a skeleton BO and 'ghosted' torpers:
your crews gonna weight A LOT when at 120lvl and you wouln't be able use all the sailor slots because of crews being too heavy, way over sub's max displacement.
other reason - if you somehow managed to use all your sub's sailor slots with 120 sailors, that would lead to a speed reduction.

now, since I have skeleton torper set and 'ghosted' (trimmed for torpedo reload cap) torper set, I can make my ss2 fly at 40knots with skeleton torpers, though with LONG torpedo reload.
Or with ghosted torpers I can have my SS4 go 40 knots and use fast reload...

One thing to remember, the more vets you have on your torpers, the more lighter they can be while beeing on torpedo reload cap.

Kind of an old thread, but my KM SS guide has math on high-repair torpedo sailors. I don't know whether the true ability cap for torpedo reload has changed since then, mind you.

What I can say with certainty is that it used to be relatively easy to cap torpedo reload on a high-repair sailor without much extra crew compared to a high-torpedo sailor. I wouldn't be surprised if this was still true.