I was browsing the TnC online store, but the descriptions of the products on sale are slightly lacking for a noob like me who isn't familiar with customs.
On to the questions
1. Are the threads at the head end of the battery tube compatible with stock SureFire heads, if yes which series?
2. What converter/adaptor will I need to use a 6P or C-series head with this battery tube.
No, the threads on the tube alone don't mate to any Surefire parts. You would need an adaptor.
The reason is an 18650 (or 18500) tube would not have enough metal left to support the threads at the head end if it's also wide enough to fully accept 18mm batteries. I think some may be doing this, but we won't. Therefore an adaptor is necessary. Currently we have adaptors that will fit our SF-Series heads and Aleph heads (Surefire E Series compatible) using TnC canisters (electronics). There is a fully E Series compatible adaptor in process and a C Series Surefire adaptor also in the queue.