The LiFePO4
batteries are some of the safest on the market due to there chemistry and should be no problem. As for the cartos I would say over three ohms, because 6V at 3 ohms is 12 watts. Now your eGo at 3.4V using a 2.5 ohm carto is only going to give you about 4.6 watts, so at 12 watts it going to be really warm if not hot. To calculate watts use this: V x V / ohms, or just go here
Ohm's Law Calculators, second section down on the bottom, enter your voltage then your ohms and hit the equal sign.