Of course I first train his secundaries and I know the elastics there. But the question was about inside training.
In line what Joey Ka is also saying, I think it's best to start with SB because it's the fastest inside skill to train.
A third goal is to use trainings at younger ages that have a high absolute skill gain, and those that have a low absolute skill-gain later. This is because the good age multiplier on a young player will give more absolute gain on a high skill-gain training than on a low skill-gain one, and you want to optimize for high total skill points in the final target skill profile.
But I'd like to have more details on which skills to train after SB.