As I know you know, you need to first define compatibility to get a useful answer to this question.
We all know that very thin layers of glass or very small inclusions of various metals can be fired together without breaking apart - at least immediately.
That's not how we define or test for compatibility at Bullseye.
We haven't done extensive testing of thin layers of Bullseye frits or powders on metal substrates in our R&E department, so any opinion I could express would be conjecture.
Can you provide more detail on what you consider "successful" firings of Thompson float compatible frit on 400 series stainless - how thick a layer of frit? What is the gauge of the steel? Dimensions of the sheet?
Then how did you measure the supposed compatibility? Simply the fact that it didn't pop off the metal?
An interesting topic, but I suspect you have considerably more experience than we do. So cough it up