1) Joe Jones is a very nice player for this cycle. To think we didn't have him in the db until very recently, when his trainer sent me a BB-mail.
From cursory glances at the DB, it looks to me like the database has a very low number of players with "low" potential (aka not-gold). As such, I'd expect that it would miss good U21 candidates who were single-position trained but had star or allstar potential, because they don't look like stellar trainees.
From messing with the salary calculator linked on the forums, and working off of the estimated potential caps on the forums, I seriously believe that for guards, it's quite possible to single position train for 4 seasons and just barely cap out a star-potential player at the end of the 4th year (assuming rounded training but no real secondary training). If someone is training for non-guard skills (ID/rebounding, etc), they might even be able to get a few extra pops before the player hits 35-40k salary.
I guess what I'm trying to say is, I think that the U21 team could benefit from increasing the focus on identifying and shepherding star/allstar potential good 18 year olds, and owners can benefit from it under the current market conditions. Being a guard trainer, I haven't really looked at what the "potential cutoff" for center-types and 4 years of 1-pos training is, but maybe someone can do a little playing around with that?