I believe you can only really start to compare young players when they at a sufficent size to compete in the position they will play. In my opinion, this is why KPP need more time because their skills, speed and ability is nullified by opponents with experience and, especially, greater strength and size. Young rucks are at an even greater disadvantage as their role at stoppages is predominately about wrestling and positioning which relies heavily on size and strength. If a young ruckmans role is more ruck rover such as what Kruezer has had the luxury to play, then size becomes less into the equation.
I think it is a good strategy for Vickery to move into the forwardline. Here he can use his height, skills, speeds and ability much more. While he will still be found out against bigger opponents in wrestling duels or even at ground level in congestion, this will assist in his development and also help the team with a bigger target while players such as Griffiths and Post are injuries/learning themselves.
So this is why we give taller players more latitude and time to development than others - its like the GWS boys playing AFL players: it doesn't matter how much skill/talent/ability you have, if you can't physically get your hands on the ball you can't compete.
