I beleive that there is a species that is freshwater, but most seen in stores are actually brackish. I've also heard that they need very flat bottoms such as sand or very fine gravel, even no substrate at all. Not sure on the tank size though. Hope that helps.
Sure! They are very terratorial fish and will fight with plecos over spots in the tank. They do not grow large but they do like spacious tanks. I Hope this helps you!