Accuracy nerds used to recommend that you cut the barrel at the tightest spot rather than at an arbitrary length. The idea is that you push a soft lead slug through the barrel, and the location where you felt a bit more resistance was where the muzzle should be. Whether this is simply theory over substance I can't say since I've had shortened rifles that shot very well, and modern barrels tend to be very uniform, but if accuracy is a primary concern, it might be worth thinking about. A 20" barrel is a practical length for the .223.
The scope you choose should be more a function of the target you shoot at rather than the rifle you put it on. If the target is small or at long range, more magnification is a benefit. My .223 bolt gun is a 20" CZ 527, at first I mounted a 4-12X Leupold on it and the little rifle felt like it had a lot of scope on it, so I put a 1.75-6X on it, which looks better, but when ranges exceeded 300 yards, I found it lacking. Just for kicks I mounted my 4-16X S&B on my 14" AR, which looked interesting, but hitting stuff a long way away wasn't a problem. If you want a low power scope on your rifle that's fine, but mount it in QD rings so you can swap back and forth with a high power scope when conditions require it.