In my Surefires I usually use 16340 LiFePO's. E1B Backup and Surefire Minimus headlamp work fine on them. Surefire recently started selling 16340 LiFePO's + charger on their website.
For the 6P Nitrolon w/Malkoff dropin I use a 17670. I prefer single cell lights.
16340 Li-co's are 3.7V
16340 LiFePO's are 3.0V so are interchangeable with CR123 primary batteries.