Yes, it is possible to cap him. Trick works because JS (for SF) is most expensive skill in BB. And if You will train his skills for example similar to:
JS = 40 JR=5
OD=5 HN=5
DV=5 PS=5
IS=5 ID=5
RB=5 SB=5
then he should be capped. His Potential Score will be above 12 (whilst ATG = 11), and salary should be above 600k.
Total Skill Points = 85 so it isn't impossible (in fact I know 18yo player who has TSP = 80).
He will be capped but probably it will be not the best method to create good player and exploit his potential in the best way.
If You want to make him really good player it is impossible to do it only by 2 position trainings. You don't need even to exploit fully MVP potential to create the best SF in game, but You need to train his skills in better way than above. I know fantastic SF who is perennial star.
23-24yo is too late to start all one-position tranings. Two-positions trainings are only JS, DV and RB (3 skills). All other skills (7) can be trained on one position.