When I first asked for the nanoboard to be set up, I came up with the 15 gallon idea because I had read that other people considered 20g a "mini reef". So I just threw in 15 g as an idea. If you want to call your 25g a nano it is up to you, but some folks would argue, but then folks argue anyway...RIGHT? And you want to put a 50 gallon sump on it ? Sure, why not? It is your tank. And personally I would not put ANY Tang in a nano. WHY?....They just look too nervous for me.
ops: I am sure the first poster who asked about the tangs did it as a joke though. 8)