I may be a bit behind the times, but as I was taught there is scientific notation which places the decimal point after one and only one significant digit and lets the exponent take on any whole number value and there is engineering notation which uses every third value for the exponents while the decimal point can be after one, two, or three significant digits. The engineering notation is what you are calling SI.
Since I learned both of them, I have no problems with either one and when I need to use one or the other, I take my clue from the context. Frankly, I see it as similar to the use of English and metric. I just know both and don't worry about the small stuff.
As for Excel, I have an older version (2K), but can format the cells with any "SPECIAL" format that I want using a number of numbers, letters, and symbols. For scientific notation the format would look like this:
0.00E+00
And the engineering or SI format code would look like this:
##0.0E+0
Of course, those are only two of many such formats. The manner of creating them is explained in the HELP files. You can even do things like having different format strings for positive and negative numbers, different colors, use the comma instead of the period, add the units, spaces, and many other options. I suspect that the same functionality is still present in the later versions of Excel, with perhaps even more options.
I have created a number of special formats for different purposes. And Excel stores all of them under the title "Special" in the format menu for cells so I only need to create them once.