This question to me depends on a few things.
The biggest distinction is whether or not you wanna train 18-20 year olds or 21+ year olds. At 21+ They can remain competitive as you train them. However this leads to the issue that you alluded on where trying to train 3 or 6 players doesn't work since training minutes, game shape, and winning games can't be all done at the same time.
Training 21+ year olds: As someone who has been trying to train 4 guys in JS all season long and stay competitive, I can tell you it isn't easy. I would train guys who need one position training because your overall roster doesn't get affected too much but you can still get training done and stay competitive.
Training 18-20 year olds: In competitive divisions, nearly impossible. I guess you can have one and train him during your scrimmage, but still very difficult. You either won't be competitive or will end up wasting minutes on non-trainees.
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.