Personally, stacking batteries is NEVER a good idea... if they discharge differently, you can have catastrophic failure...
You appear to be using a Nautilus tank, so you'll not be pulling huge Amps (based on the factory coil resistances), so the danger is greatly reduced, but I would use an 18650, rather than run the risk of stacking batteries.
TrustFire has a bad rap, too... inferior products and poor reliability complaints are all over the Internet. I have a couple of their flashlights which are decent for a cheap LED light, but the batteries were garbage (fast discharge, won't charge, and don't last to the extent of their rated mAh)... and their "free chargers" were just as bad... at least that is my experience with their products.
I highly recommend NiteCore i2 or i4 chargers... you can pick them up (Amazon) for <$15 for the i4 series. As for batteries... I trust Sony and Panasonic for safety, durability, and long-lasting performance over any Chinese off-brand battery. You can usually pick up Sony, Panasonic, or other "name-brand" batteries for <$10 each (or less if you buy two or more at a time).