Long nose are known for munching on shrimp and small fish. It might not be a wise idea to house one with a small goby.
When it comes to mandarins, they are extremely hard to care for. For the most part they only eat pods
and need an extremely large amount to be successful. I kept one in my 55 reef for a tad bit over a year and spent a fortune trying to keep the tank stuffed with pods
. This was with it eating mysis shrimp as well. Not something I would recommend.
What I would recommend would be a dottyback. Great fish for a tank this size. I loved my indigo dottyback, had great personality.