I downloaded your adapter and merged the STLs in SketchUp. It looks as though the hexagon sits a bit higher and juts out a bit more on top of the alu plate than the buda. Your adapter accounts for the difference of the hex sticking out to allow the flexistruder a flush mating with the plate. Thanks for sharing the adapter. 
After muddling through the above, I started comparing the flexistruder to the hex extruder. Seems to me that it needs a “stem” under the hobbed gear to ensure the softer filament is fed properly… Depending on the tolerance before the hotend assembly, maybe another little stem there also. Should be a pretty easy mod.
When I can get my hands on an extra hexagon, I’ll take a look… right now, I’m having too much fun putting prints through the TAZ 5.