You could be right about mixing. I think when I used the dot 5 before, I flushed out all the old stuff. I like the silicone fluid because it doesn't absorb water, which destroys cylinder bores and causes things to leak or sieze. Also, dot 3 & 4 can become dangerous if they aren't replaced occasionally, since absorbed water can cause a lower boiling point for the fluid, something you don't need coming down Pike's Peak.
But I'm reluctant to use dot 5 now without some kind of assurance it's OK in these cars.