Please please please talk to Paul B about "why" Mandarins eat so many copepods. Has to do with their unique digestive tracks, that are unlike most other fish.
I won't go into the sordid details but I will tell you this....I killed 2 Mandarins and a Scooter Dragonet trying to do exactly what you are doing. I also ruined a 3yr old tank. It was an experience I'll not forget and it's been over 2yrs.
If your tank is not large enough to sustain a thriving copepod population your Mandarin will not survive long on prepared foods alone. No matter what you tell yourself.
That is why I intend to give him a constant supply of live brine shrimp. Not frozen, I have a scooter blennie who is actually very healthy, this I am certain of because of his coloration. He is very dark and solid looking. I understand your concern,but it has been done successfully. Also, I am confused. As crazy as it sounds I got my scooter when my tanks was one month old he has honestly been very healthy the entire time.