When I was working in a shop back when dex cool came out, we were all told the same thing. Dont mix extended life with the green stuff, but no one could tell us why.......
We decided to do an experiment. We pulled some dexcool and some standard green coolant mixed them in a clear jar and left them on the shelf. After months and months nothing happed. We put broken pieces of aluminum casting from a smashed water pump, we added cast iron pieces, steel chunks, even put part of those gm plastic /rubber intake gasket.......anything we colud think of to try and get this bad thing to happen by mixing coolant. We even heated up the coolant mix ( minus metal) in the micro wave to boiling point. Nothing ever happened, no solids, no cloudyness, nothing exploded...haha
I never understood why it was "bad" to mix them, But in our redneck testing found nothing out of the ordinary by mixing them. It wasnt long after that, they came out with coolant good for all makes and modles, so we gave up.
My limited and maybe worthless 2cents
Brian