A player will be capped when he reach a total skills sum on a certain position. If your player is a guard, he will reach the cap if you train him on OD, JS and JR and he could be capped at let's say 100 tsp. But if you train the guard on SB, Reb and other skills not "directly" related to the guard role before OD, JS, JR, you'll see that the same player can reach even 110 tsp.
Regarding OD level it depends a lot from your opponents, in the Italian first division lvl 15 is not enough but as far as I know it should be a good value for your division/nation.
1990-2022 Stalinorgel - https://www.youtube.com/watch?v=pV-Xppl6h8Et