the 21 year old and 22 year old are miles ahead of the 18 year old in a sense.
the 22 year old and 18 year old both could use more driving which would help with elastic effect for outside D....
so 1v1 forwards would be helpful for that. I'd do that, the OD and passing for now then.
the 6'4 guy being on the shorter side, and you wanting to make him an SF.... Now would be the time you'd want to build up his Big-man skills. But that doesn't mesh with the other 2 guys.