Hey Jon, the shape of each boot is different enough that Dynafit had to make separate tongues for each model, however with a bit of modification I have seen a few people cross models and brands with success. Keep in mind, any modification you do to a tongue will void all kinds of warranties though and I don't necessarily recommend it.
I recently bought a PDG Instep Buckle. I wanted to try something like a rivet to attach this to my boot but the pop rivets you can buy at home depot didn't really work. They don't leave a flat surface so it would dig into your foot. Do you have any recommendations for how to attach this on the cheap? Links please if you have them. Thanks.