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.
First off, thanks for the reply! Its really helped thing along, I changed the interaction text and put the sphere in so now fire deities give out the secret, which is good. As do dragons, funnily enough. They have fire spheres. Didn't think of that, but its pretty cool.
The fireball throwing works-adventurers can learn it, and NPC Flame Mages will roast attackers.
Turns out now though that they aren't passing the secret down. When they seemed to be doing so earlier, I think it was the necromancer connection.* Seems like there's no books or apprentices like there are for life-and-death secrets; is that normal? Does it require another tag to make them do that or something?
*EDIT: It is. My attempt to stop Necromancers and Flame Mages from overlapping didn't work. For some reason, the hybrid mages teach both secrets to their apprentices and write them both down, but pure Flame Mages do neither. Still, its a start.
EDIT2: Fixed both of those issues/worked out that they weren't issues at all. It all seems to be working nicely now, as far as I can tell. Time to see what else I can play with. Thanks for the help-I'll charbroil a monster in your honour :3