Man If your going with a Glenndyne Wheel. Go ahead and do a 23. For the money if you decide to upgrade later on there might not be a market for a used 21 of there price. But if you want a 21 no fender modifications need to be done on some fenders. Some need to be drilled out a small amount but others dont. Depends on your fender. That is what I have read on multiple posts. just my .02 but you do have good taste. I just got off the phone with him.
Just make sure to hike up the rear of the fender as much as you can while tightening up the rear mounting bolts.
Having the tire being able to turn freely on the jack isn't enough. Dont forget the tire expands at speed.