I had a Banggai in my 24 gallon nano. I bought him from Tropicorium (bred there) at about the size of my pinky fingernail. I thought there might be trouble with my 6 line, but everything went smoothly. He grew quickly and was generally not aggressive although he could hold his own.
Mine always chose a position in the open toward the front of the tank. He wasn't a picky eater at all and particularly enjoyed the custom made food they sell at Trop. Mine was a very enjoyable reef inhabitant, and I wouldn't hesitate to get another.
I'm not sure how they do in pairs as mine was single.
They are cool, but if you happened to buy two males, one will kill the other. It's best to buy them in three's or five's, that way you normally end up with a pair. The LFS should have told you about that. I hope you have one male and one female, you won't find out until they get more mature, I bought three and it took one year before a male killed the other male. Best of luck!