this is a tough one over the web. The surfaces will polish but not really wear in the short term. It should be a little snug but we may judge "little" differently. Maybe someone else can help more..sorry.
There's quite a few ballistic calculators available. I started a collection of info here. Maybe something there will help. Heres some stuff i bookmarked a while ago loadammo.com SWAT Chuck Hawks Your going to find that 16" is just fine :)
The carbine length gas system is tougher on the weapon then rifle length for sure. The reason for custom buffers and springs regardless of caliber Sorry I dont know anything about the 6.8, maybe others members do.
No first hand experience but I have read numerous times to avoid it. I have settled on PRVI Partizan 168gr match as my cheap ammo. Great bang for the buck. Not sure how much of a price difference there is between Prvi and Wolf