Nuno, usually the small internal gas tank will allow to perform 3 starts. So to answer your question, NO, the ecu will not burn all the gas from the small gas tank. If you don't want to have the internal tank in the airframe for saftey purpose is ok. Just use external source to ingest the propane for the start up. Don't forget that you still have to install the gas selenoid, it will regulate the pressure in the start up secuence. A lot of us don't like the internal tank for the same reason.

Good luck
Alejandro