The way you have it set up doesn't make sense to me. I will tell you what makes sense to me, and I don't know if I am agreeing or disagreeing with popular theory.
If you are an ISTJ, Si-dominant, your shadow will use Se. I have come to this conclusion by noticing that my Ne is always connected with Ni. I get small Ni flashes and 'feelings' to accompany my constant Ne.
Sure, it looks nice to line up all eight functions and reverse them, but I don't think it is that simple, and I don't buy it. Your functions are paired together, you use Si WITH Te. You use Fi With Ne, in a Fi-dominant manner. I think it would make the most sense for a shadow not to be reversing the letters, (ISTJ > ENFP), not to reverse the functions (ISTJ > INFJ), but to reverse the introversion and extroversion on each of your four dominant functions, which just happen to be your next four functions, in order. I think within each group of four there are two pair, the lower being inferior, and within the whole there is one pair (comprised of two groups of four functions each), the lower of which is inferior and rarely switched to.
That would mean you are on the right track by saying you switch the middle two letters- but you are not quite right. In your example,
Si>Te>Fi>Ne> Se>Ti>Fe>Ni becomes
Ni>Fe>Ti>Se> Ne>Fi>Te>Si.
But in the bold sections, I see no reason why your shadow would 'reverse' the order of your functions, making the weakest one dominant and the fifth one weakest. It makes more sense to invert the Introversion and Extroversion of each function, and then apply it. That means....
Si>Te>Fi>Ne becomes
Se>Ti>Fe>Ni.
In this manner, an introvert's shadow is extroverted, and vise versa. It uses the "other part" of the dominant faculty and so on down the chain. The formula is to reverse the first and last letters, which is why I said you were on the right track.
ISTJ > ESTP
With your method, my shadow would be an ESTP. I...really don't think it is. I have only one time been so crushed that my personality totally changed. I used Ni and Fe at the time, constantly seeing long-term visions of peoples likely actions and how I felt about them. It was very, very different from anything else I have experienced.