Depending of the year, the needs for the U21 can be very different. One can easily train a C with a starter potential and he will ready to help if they aren't enough good player around (Brendon Atama this season). At the same time, players with more potential can still help the team even though they don't focus on primary (Ximen Quanjiang). One player I like and who is well trained is Chen Guangjie, he doesn't have enough potential to play C in the NT but he will be a great DII center and he may even play in MBBA.
To me, the choice of how a player is trained should belong to the player's owner. We can only suggest a training but in the end, I always advice the owner to follow what he thinks is the best for his team and the player while keeping in mind what is best for the U21. Diversity is interesting too and we can do without another Gabriel.