Creature fights target creature
You are overthinking it. Two creatures fighting each other as result of a spell or ability means exactly what it says, and nothing more. Power and Toughness of a creature are always one certain value, namely the printed base value plus all applicable modifiers. Two creatures fighting each other is NOT combat, and combat-related abilities don't matter. The damage they deal to each other is NOT combat damage, but simply damage with the respective creature as the source.