we're not saying that 1 pop per 1-2 weeks is not possible at age 21, we're saying that a short player will get that when training outside skills at 21, and a big player will get that when training inside skills at 21. However if you train the short player on inside skills or the big player on outside skills at 21, you'll get 1 pop every 2-3weeks if you're lucky.
There's no need for a new debate, I'm sure you have a good communication with a few canadian managers. However seeing that 2 players on your team (that's 1/8 of the team!) come from a manager with whom you had 0 communication in the past is worrying. You've got to be proactive before they reach 21yo and contact all the managers of players with good starting skills and reasonable potential as soon as they are drafted. As I said it's not that hard for the U21, there's only 5 active leagues in Canada + a few ghost in d3.
If there's something like 4 or 5 players worthy of training for the U21 in each league draft, that's like 30 managers to contact. It's really not the end of the world. If you wait until the player reach 21, or only communicate with the ones that contact you first, you end up in the current situation.
I'm not saying that's what you do and I don't know your previous history with zmtomako, but if that's what really happened I understand his frustration and his response.
As for training speed, josef_ka data is almost a decade old by now. The principles remain valid but cross training was added after that, and the basic information is right there in the game manual:
"younger players will improve more quickly than older ones, and taller players will train faster in some areas while shorter players improve faster in others. You'll have to take advantage of this information to optimize your training routine."
I would be really impressed if you manage to get 9-10 pops in one season at age 21 while doing "out of height" training