Ok, a quick run down of compatibility, there are adapters for most main stream batteries to fit whatever cart/atty you wanna use. but the 901 atty fits on a kr808 battery just fine, but the reverse is not true. Kr8 cartomizers require holes in the battery threads to allow air flow, 901 atty has the whole on the side so the batteries dont have any holes.
As far as mods go, do your research if your looking into a bigger battery. I am personally awaiting my indulgence from eastmall. The indulgence is pretty cheap ($60) and comes with 4 adapters. You can use a 510, 801, 901, kr808d-1 carto, or 40x atty on it. The price includes batteries and a charger, and for 10 bucks more you can make a pass through out of it, or 20 bucks more you get the extended cup and batteries to make it 6v.
So just do some research and you will find what your looking for, if you have any other questions im sure we would be glad to help
