You might try adding [IS_SPHERE:FIRE] between the NAME and GOAL tags. Take out the ADD_TAG:FIREIMMUNE. Unfortunately, we can't currently add that one. If you want to make your flame mages fireproof, the only way to do it right now is with a transformation, which gets a bit more complicated. You don't need a separate interaction for the fireball. Get rid of the second interaction and replace the part of your secret that's supposed to add the fireball with something like this:
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Hurl fireball]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_TYPE:GRASP]
[CDI:FLOW:FIREBALL]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:VERB:point:points:NA]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:30]
I think this will get you as close as possible to the secret you want, unless you'd rather go ahead and transform the flame mage into something fireproof. Fireballs shouldn't ignite them very often unless they set the entire landscape on fire and walk into the flames, I believe.