My opinion on who is U21 material (assuming all are Australian).
Shane Goodes very unlikely but you will know very soon, sometimes there are few big men to pick from.
Tobias van den Hout one season to go also very unlikely, also needs a ton of rebounding.
Wilbert Phillips has horrible secondaries but rock solid primaries and great height, he is a great chance.
Duke Dodd and Homer Knell have too low potential I would not train them unless you plan for an entire season of rebounding (would be good for Goodes, Hout and wouldn't hurt Phillips) and they can be trainees four and five.