Problem one: wrasses was added first. Therefore the mandarin is an intruder in his territory. Aggression follows. Maybe for a while. Possibly stressing the mandarin to death.
Problem two: they compete for the same food.
Problem three: mandarins, even trained on other foods, still need to be able to eat live pods. Wrasses could easily out compete.
Problem four: pod populations don't rebound that quickly. May need to dump in bags of pods to supplement.
Good luck! Let us know how it goes.
....Salt Creep Is Everywhere!....