The thing about that is he'll have an extremely advanced Magi to learn from. i'm of the opinion that all the really good ones develop their own method as it were, besides if he only takes those with talent it would be reasonable to assume he has methods of telling what talents they are capable of and someone like Shirou would be too interesting IMHO.
And therein lies a problem twofold. As I've stated before, I find it a bit hard to believe Zelretch would take Shirou. You can't just look at a person and say "Oh, that boy's got talent!", especially a SIX-YEAR-OLD. Even when Zelretch picked out some apprentices, he took the BEST, that had shown themselves to be so, he didn't hand pick them himself.
Additionally, Shirou's method of Tracing is based on Kiritsugu's method of using magic, which is one that no sane mage would ever thing of using (it puts you constantly at risk of killing yourself). Zelretch would teach him the conventional way, as there's no way he would know to teach Shirou some self-destructive method for no reason, and Shirou would never have the opportunity to learn any other method (unless they run into some alternate version of Kiritsugu... but I can't see that working out well either).
NORMAL magi take years and years to develop properly (think decades), and Shirou would have
ten years. It just wouldn't add up.
on a side note, what i meant by 'instinctive capability' was that with normal Circuit knowledge he could have originally done all the Tracing on his own however with Kiritsugu's teaching it not only sped up his tracing ability but gave him at least three problems with tracing and not Reinforcement that he eventually got around. this is just personal speculation to make things more plausible. please, point out any holes in my logic with this scenario.
...I think you're under a mistaken impression.
Kiritsugu's method of teaching? It's the
entire reason why Shirou can Trace like he does. Shirou literally invented his method of Tracing based on Kiritsugu's way of using magic. If Shirou were taught the conventional way of Tracing, it would lead absolutely nowhere, as they very different in what they do.
Basically, as I said, Shirou could not possibly learn to Trace weapons the way he does in canon if he didn't learn his bizarre way of using magic, since its principals are based on that method.
What i'm trying to keep in mind is that something like this has huge repercussions, not only in character ability but in figuring out what's a part of them at such an early point, as i'm still under the impression that he was born with his Reality Marble such a thing should IMO make an impact on his Tracing ability especially with such a teacher.
i know i'm forgetting stuff though so i'll be heading to bed soon.
Uhm... no. People aren't born with Reality Marbles. I'm sort of hazy on how they're created, but I know that people aren't born with them.