Thank you for all the suggestions. My main criteria is portability as I would love to be able to carry the stick around all day in my shirt pocket( I dont continuously use it, but would like to know its there, when needed). The reason I had gone for the 650mAh over the 1000 and 1600 versions was the shorter length which ensured it did stick out like a chopstick from my shirt pocket 
I'd prefer sticking to one or even 2 650mAh's but not sure if the protank or nautilus minis would get enough power from the smaller battery to provide their full vaping efficiency? If the 650 battery is going to be the bottleneck, then I guess, I'll have to start considering keeping the longer battery loaded kit in a pocket other than in my shirt lol
I'd prefer sticking to one or even 2 650mAh's but not sure if the protank or nautilus minis would get enough power from the smaller battery to provide their full vaping efficiency? If the 650 battery is going to be the bottleneck, then I guess, I'll have to start considering keeping the longer battery loaded kit in a pocket other than in my shirt lol