You would not be able to use the 18650's with the kick in the 3.7/6 Saber, you would have to use 18490/500's with the kick installed. But you can always take it right out and use it with the 18650's when longer battery life is desired.
And You would be correct, just drop in the kick, drop in the battery, and your good to go, simple
Now as far as if it keeps it's power, yes. I have only noticed(on EVERY mod, not only Sabers) that when the battery drops to low, you get this hit, fire, hit, fire as your touching the sensor(or pushing a button). But again, this is right before the battery dies only. 
And You would be correct, just drop in the kick, drop in the battery, and your good to go, simple
