The company was responsive to support before ordering to ensure compatibility.
The interfaces can be adapted... it's just voltage and all you have to do is step up or down.
It isn't quite that easy. The high-output 18v batteries can dump over 100 Amps for short durations. Stepping that kind of power up or down isn't really possible in any consumer device.
The good news is that Dewalt 20V and Milwaukee 18V are actually the same: they are both 5 li-ion cells. The no-load voltage of a li-ion cell is ~4V, the voltage under load is 3.6V. This is why you can use Milwaukee batteries on Dewalt devices and vice-versa.
So some batteries protect themselves from being overdrawn, others do not.
M18 batteries, for example, are dumb and will let you discharge them way too low where they won't take a charge anymore.