People with manderins let them eat pods ( little white bugs that live in the tank ). Very few manderins ever learn to eat prepared food and for this reason some new reeferes loose manderins to starvation having bought them not realising their needs. But a few manderins learn to eat frozen food.
To have a manderin the tank needs to be of a size and nutrient level to support a big enough pod population to support the manderin.
With your Goby, it is unlikely your sandbed will ever supply a major part of it's diet, because the emphasis in a tank must be on keeping the sandbed and tank clean. You can only have a large population of critters in it, if there is a lot of organics in there for the critters to eat. But this is not such a good thing for the tank generally.
However, Pies is the man to talk to about this, he has a refugium that supplies a big part of his fishes diet, from critters that breed in it. If he is not reading this thread you could drop him a pm.