1. I probably won't. I don't think there has been a single manager who has intentionally tried to piece together a roster that suits more than one tactic without skimping on its effectiveness. You pick your best players for the main offense you want to run, and if those players happen to work well in a different tactic, than that's fantastic. More than one tactic has been played seemingly every season, but the chosen roster is best at only one tactic.
I REALLY hate all this talk about "we must be prepared because GDP...." and stuff of that nature(certainly not aimed at you. I have no idea if you were talking about GDP or not with this question). BB is a game with a ton of active variables, so altering how we run things because of an additional, new variable is not practical at all, until it is proven that GDP can size-ably alter the game. It would be like finding out this season a change has been made to make ID more effective, making LI/LP less effective, and then picking a roster for Motion/RnG, even though the LI/LP roster is better. You don't know exactly how much more effective ID is, and how much efficiency LI/LP lost. Same concept here.
2. I would absolutely like to plan a player from 18/19 to have a specialized role for the U21, and possibly NT. Like you said, the owners cooperation is critical. If they are not 100% convinced with the plan that I lay out for them, than they don't have to follow it, and it will not be held against them. If I was given 5 guards of similar talents and potential I would look over which one would have the easiest path to the specialized build(among other things like owner nationality, offsite participation, overall participation, and willingness). Having five guards of similar build at age 21 is useless. You don't want to ever have more than two players on the team, at any position, that have a similar build, because team effectiveness goes down (you wouldn't run a team with 5 PGs, 5 SGs, 5 PFs etc.). So if that happens, three managers who have trained their player very well are S.O.L.
Player skill diversity is critical. It gives you so many more options, and makes it so much easier to run a team.