[...] any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.
The Elemental Adept (Fire) feat says (PHB, p. 166):
Spells you cast ignore resistance to fire damage.
Would these two stack together to allow the user to completely ignore fire immunity (so Fiery Soul treats the immunity as resistance, and then Elemental Adept further allows you to ignore that)?
Or would Elemental Adept be useless to a pyromancer at 18th level, as they can already ignore resistances?