Well, I did some research elsewhere, and have managed to answer a few of my own questions. So for anyone else that was wondering (or myself if i ever forget lol):
1) SCA: System Coolant Additive, DCA: Diesel Coolant Additive. Apparently the terms can be used interchangeably to describe, in generic terms, any additive package used to address coolant cavitation issues in diesel engines. The terms can also show up in specific formulations of additives, however, which led to at least part of my confusion. Examples: DCA-2 & DCA-4, which are two unique formulations of additives, each made to work with particular engines. I haven't found any specific formulations that use SCA in the name, but that doesn't mean there aren't any (are there?). Also, many specific formulations don't use either nomenclature in their names... VC8, FW16, etc.
2) Still don't know the answer to this question, but as I've learned that excessive levels of SCAs with coolants that require them can promote scale, it would seem logical that using any SCAs with coolants that don't require them could at a bare minimum lead to unnecessary deposits. I'm still curious if there are more extreme compatability problems.
3) As long as the test strips are made to measure the formulation of SCAs you're running, you should be able to go off their min/max values. And max values are important. Excessive SCA levels are problematic as well.
Here's one link I used that I found helpful in researching the subject. There were others, but I forgot to save those: