I'll group the skills in tiers
1. IS, ID, Rebounds
2. SB, PA, JS
3. DV, HA, OD
4. JR
IS, ID and Rebounding are absolutely necessary. SB, Passing and JS give your C more versatility and while not absolutely necessary are still quite beneficial. Driving, Handling and Outside Defense are nice to have, and Jump Range is almost useless for a C.
can we see this 21yos skills? starting training at 21yo is usually not the best idea unless the player is already quite developed.
Obviously you don't have a clue my friend

JR is MUCH more importan than DV and HA,and quite important itself..
I want what all men want...I just want it more.