I think if they resemble thier smaller cousins, a minimum tank size should be a 75 gallon or a 125. 6" is a lot of fish, especially when they can move that fast. Plus, you will definately need a group of them, the more the better.
Let me give you some examples:
Halfbeaks - small fish, but every one I see in under a 55 gallon after a few months has a broken jaw. They are too spooky for small confines.
Freshwater barracudas - basically predatory tetras, again, not a huge fish, but in under a 55 gallon you are immediately asking for trouble.
Giant danios - can live in a 30 gallon, but they truly thrive when you put them in something like a 55.
All I'm saying is don't let your enthusiasm override sound judgement. I really wanted a giant gourami, but a 75 gallon is too small, so I went with a different species instead. Sorry to burst your bubble, but they will probably be really difficult to find anyway. Best of luck, let us know how you make out.