I have seen a 21" front on here, and on youtube. I forget the dude's name, but he has Roland Sands Apes on his bike and it is Olive/Black (just like mine !!!). As far as the 18" rear, haven't seen it yet, but I would LOVE too. My bum is kinda big and I don't want to give the girlies too much of a show while rollin' down the road (they gotta pay for that, lol)...
what else are you going to do? a lot of the top builders are doing same sized wheels front/back. using skinny tires. i think the nightster would look good with an 18" out back and a bit more fender-bob. thats where i'd like to go with mine.
Tweakin I believe did it to his nightster. Search him and you'll find it. He no longer has it, but his pics should still be in his gallery and in some threads.