You can keep these fish in the size tank you mentioned while still young and small. They will need a much larger tank as they grow out to adult size. Those fish grow fairly large.
are you talking about these: Gobioides broussonetti
dragon eel/violet goby?
they are monsters and get to be about 12 inches long, they aren't predators, just huge
I have a trio of archers in my tank and they are are about 9 months old and average 4.5"
Shikkapow - I was just curious about the age of your archers...are they captive bred? if not how did you find out how old they are?
Also I have been out collecting wild violet gobies and have found some as large as 16-18". They are pretty cool fish regardless of size, but are sometimes difficult for beginners to keep.
Check out www.fishbase.org for info on fish species their habitats, and diets. researching how the fish live in the wild the best way to learn how to keep them in an aquarium.