When you are starting out as a fire ninja, the only other fire ninja you will have near the beginning is Choji who makes a pretty decent tank once you have him at two stars. But once you complete chapter 4 of the main plot, you will be rewarded with Anko. Put her in the back ranks since she can act as a support with Fire Style Barrier: Red Lotus Inferno. What this will essentially do is amp up the attack and ninjitsu of all Fire Attributed Ninjas. While it may be tempting when you raise the barrier. DO NOT RELEASE THE BARRIER ON THE NEXT TURN! You can utilize the barrier for your main attack such as Fire Style: Dragon Flame Jutsu, which will be insanely damaging especially if you have the Fire Style Enhancement. As for the extra space in your lineup, I suggest you use a character that can create clones, Naruto or Kakashi would be a good choice. If you want to release the barrier, make sure that your opponents are weak enough so that you can finish them off with that one hit. It is also good for disrupting the opponent if they are going to use a jutsu next turn.