Note that the first ref cited above suggests using aquarium salt, which is not suitable for any brackish water fish. Marine mix should be used to prepare the water for the dragon goby as for other brackish and marine fish. High specific gravuty does not seem to be required, light brackish seems to do, 1.003 - 1.008.
Freshwater fish are not suitable tankmates under the proper water conditions for these fish. That info is mistaken as well.