Banjo's are known to be very shy/nocturnal fish.They are actually considered a good community fish,but will eat any small fish that they run into at feeding time.They tend to average out about 6 inches. If you keep your Goldfish tank more tropical than cold,I guess that you could try one in your tank.I personally think that between their shyness,and the Goldies being so aggressive at feeding time & such, and just in their general behavior,that I probably wouldn't mix the two.
The Banjo would stay well hidden in the tank most of the time too,so it definately wouldn't bea fish that you'd see too much.