The reason I asked, I did a DNA40 upgrade to my waidea and a spring loaded 510. (I linked to my post in the other thread earlier in this thread)
I've done 2 Flasks so far, an infinite and a waidea. The waidea usb charger is on the bottom center behind some kind of grommet, which I'm told can come right out but I had no clue how to get to the usb charger to upgrade it. The infinite had a complete sled that when removed had the usb charger on the very bottom of the sled that comes out of the flask making it easy to replace (the infinite had a 1amp usb charger already so there was no need) I do not know what the waidea usb charger is rated at, but it's been speculated 500ma
I have no knowledge of the kangxin.
I will also say, again on my waidea, the fdv v4 shorty spring loaded core threads right into the single air slot 510 connector on the waidea with no real problems except that the retention "nut" on the fdv v4 is press fit and crimped on and mine came loose so I had to solder (think plumbing sweat copper pipe) it back together.
But again my information is solely based on my knowledge of the 2 Flasks I own which are the infinite made and waidea made. (sorry)
Keep us posted with your process and success