OOO...the orange spot goby is nice. I had one but it died :( Dunno why. It was paired up with a pistol shrimp. The shrimp is still doing well, and I'm going to get another orange spotted goby and hopefully they pair up as well. The shrimp digs, the goby feeds it :)
The back color of the tank is personal preference. My 45 gallon is painted black, and my 10 gallon just has a blue tank cover thingy taped on the back. Both seem fine, but I think I prefer the black.
I think a 6line needs at least 30, and might be aggressive. But I'm not a fish expert. I just go by what others have said, and also by liveaquaria.com. If you get it anyway, maybe get it last, to let less aggressive fish establish their territory first.