Diamond gobies don't need anything special in their diet, they will eat the same stuff as your other fish, and they are compatible with mandarins, but unless you have a mature tank with a fuge that produces a lot of pods, I wouldn't get one, or it would quickly starve in your tank as most mandarins will only eat live foods. If you can find one at your LFS that eats frozen foods (and make the LFS show you it eats frozen), then you'd be okay.