do I just put that in a .txt file and add it with the rar? (none of my emus use this type of file)
just note in the readme to add that to an emulators/flashcarts "game_config.txt" file if it has one, if it doesn't have one then it's most likely not compatible with it, and then the only way to fix the problem becomes to change the header back to the fire red header.
By the way, this is the standard fire red game_config entry:
# Pokemon: Fire Red (E/U)
game_name = POKEMON FIRE
game_code = BPRE
vender_code = 01
idle_loop_eliminate_target = 080008c6
flash_rom_type = 128KB
All PSP emulators, a number of GBA flashcarts, and almost all mobile emulators use this file.