Below .7 ohm s the contact point at the top of the battery heats up and can melt the fire button. Rob is coming out with an upgrade kit that will fix this.
Other batteries can be used but you need to make sure you understand what the max continuous amp load is that they can take and that your builds do not exceed it. The device was built to use button top batteries, flat top can be used but you more likely to get sparking and oxidation, plenty of noalox will limit this.
the AW IMR batteries are know high quality that have plenty of performance and are what it was designed around.
As far amp load you want to stay under 70 to 80 percent of the batteries stated max. The AW IMR 18650 1600mah has a max amp continuous discharge of 24amps, the AW IMR 2000mah has a max discharge of 10amps.
Either battery gives plenty of headroom with a .7 ohm coil. A .7 ohm coil will pull 6 amps. however if you do the low ohm kit upgrade and start running .3 ohm coils you would need the 1600mah as a .3 ohm coil pulls 14amps.