Aloha,
It could be just fine
OR the Midas could come in and kill the Firefish. Similar size and body shaped fish can be very aggressive towards each other. To be sure that everything works out positively I would prepare and do the following:
First, I would keep the Midas in an acclimation container for 2 days to be sure it gets accustomed to being around the fish before it's set free.
Second, I would feed really well for those two days so the Midas will see that it needn't fight with the Firefish to get its share of food.
Third, I would keep my training stick ready and use it to let the fish know that you are the Alpha, the boss.
To read more, look in the Reefkeeping Tip "Secrets of adding new fish...."
Mahalo,
Mark