Any question answered...

No, you're right, you can. But like incest and country dancing, just because you can doesn't always mean you should :-)

I think on most hubs it's fine, but on some the fixed hub has fewer threads. It depends on the hub, though. If I were buying a bike new I'd want to just get a fixed/free hub in the first place instead of fannying about worrying over whether the freewheel was ok or not. But, y'know, that's me, other people don't mind so much.

The genesis you linked is really nice, but I bit over my budget.

To add something else to the confusion, I just found that in the specialised website, the langster is listed as having a flip-flop. No idea if there are any differences with the langster they sell in Evans:

http://www.specialized.com/us/en/bikes/road/langster/langster