Very interesting question, unfortunately the u21 world is dominated by triskill bigman (Center with IS 17+ ID 17+ and RB 14+ that cost around 80-100k), so if i had to chose between a 17-17-14-10 (IS-ID-RB-SB) with no secondaries or a 15-15-12-8 with 10+ in JS-HN-DR and decent OD and PA, I would probably go with the first one
But simply cause with the enormous pool of players that Italy has, we were lucky enough to have the opportunity of "testing" both out, and the guy with more primaries was simply better.
Obviously things would change if i had a guy like Mastrodonato, who was 17-17-15-10 guy with 11 JS and 11 DR, although we had Padula and Stefanelli with more inside skills, Mastrodonato was a nailed on starter every time he was in decent GS.
With this being said, I wouldn't force anything, I would layout to the manager multiple training plans, the pros and cons of each one and I would let him decide what path he'd like to take
On exterior players (PG and SG) you don't have this problem since my players rarely have huge salaries, and it's easier for them to be balanced since 1on1 provides a good amount of IS training