I definitely agree that height would slow you down, but the point I was trying to illustrate is that if you ignore height, a center can be trained in guard skills and receive just as many pops as a guard. So it's not really that guard skills train faster, its more than One-on-one pops really quickly which is a guard skill and so it might seem like guards get more pops, but a center would get a good number of pops if they trained one-on-one as well for a while.
I definitely think you can get 13 pops at least with an 18 year old trainee and just a level 4 trainer. As long as the height is decent, one-on-one training is surprisingly quick at training driving and handling, and it trains multiple other things as well. If you had some high sublevels you could very easily do it.