I'd lie if I told you exact numbers about pops. I'm not sure about it, but it's something about that you wrote.
Optimal training = minimum posts covered by training... so i.e. 1v1 optimal is 2 positions trained. For IS 1 pos.
The reason why most suggest starting with 1v1 is because it's fastest training. And more important, it builds elastic effect for other skills. Check out bubbles buzzerbeater account on YT, for more info.
So elastic effect is more of a reason to do 1v1 early. With age, your prospects start to slow down in training. And elastic effect kind of mitigates this.
1v1 early is also good because you can start with 6 18yo players and flip 3 of them on TL when you want to swap to single position training later.
But don't forget, it's your team, experiment, do your own stuff. Otherwise it might start to be tedious for you.