The reason the scooter doesn't eat amphipods is sheer physics. Look at the size of the amphipod compared to the mouth of the scooter. They are too large to be ate whole, in which case, scooters/mandarins do not have the mouth structure to be ripping and shedding amphipods to get them down. A sixline wrasse however definitely does, but they will also eat your other pods and starve out the scooter. The best option is to live with them.
By the way, Firefish ARE NOT gobies. Firefish are in the Ptereleotridae family, not gobies (gobiidae), commonly named Dartfishes. Which obviously includes the Dartfish seen in the hobby, some times called the scissortailed goby, and there's also the Blue gudgeon seen somewhat frequently.