I want a Violet/Dragon Goby?

Here in a few months I'm planning to set up an Aquarium for a violet/dragon goby. I know they need clean brackish water between 75 and 80 degrees, and that they might grow over 2 feet, they are peaceful, don't eat small fish, and that they love algae, blood worms and brine shrimp. What i need to know is what size tank should I get for it. I know it should be longer than tall, but how many gallons. Would 25-30 gal. be fine? Also what are some good tank mates. I read that fast moving fish aren't a good tank mate because the eat the food before the goby can get to it. But what about a small syodontis, a small bottom feeding catfish. It's mainly nocturnal, so it feeds at a different time than the goby, but I don't know if it will survive the brackish water. Are there any other fish, amphibians, snails or crustaceans that i can keep with it?