if he is a C the skill sets would be inside scoring|+ rebound+ inside def+ block shot?
Correct.
and out of balance: big difference between these skills or training in a skill that the player is not suited for? (ex: rebounding for a 1m80 player)
Not really.
Let's say you have a player really good in one of his relevant skills, but the other three are lagging.
That highly-trained skill will train more slowly than it would otherwise, and everything else lags until you achieve a better balance.
NO ONE at this table ordered a rum & Coke 
 Charles: Penn has some good people 
 A CT? Really? 
 Any two will do 
 Any three for me 
 Any four will score 
 Any five are live