Even the 5.56 from a 14.5" requires a flash hider. What wasn't said was whether they were removed for the video. It makes a nice Hollywood statement on video to screw off flash hiders and let the flames roar.
There are also a lot of vids on youtube that show exactly which flash hiders do a decent job of completely eliminating flash. There are quite a few on the market now, and plenty of compensators that do it well, too. For a 6.8, you will need the larger .308 versions, as the 5.56 won't let the bullet pass thru. Most of them come with the larger thread.
Barrel length should be optimized for the use, not vice versa. It doesnt really say much to repeat it's an all out gun. That's more chest thumping about a credit card limit than defining how it will be used. The 6.8 was specifically designed for assault rifle use from a 14.5" barrel out to 450m. 16" is the actually the most common barrel, with midlength gas. Some are making 18" to emulate the Squad Designated Marksman, it really doesn't add much for the two inches, just a few dozen more feet per second in speed. Using different ammo will do that, but it still doesn't say what the target will be at what range.
If it's a long range, 400 to 600m gun, get a 6.5G and you get the benefit of the ballistic coefficient working to keep speed up. Works great on paper, and takes smaller game with ease. If it's to be used under 400m, it's not really a paper gun, it's a hunting rifle. That's where the 6.8SPC shines, as it has more speed with the slightly lighter bullets, out of a 16" barrel that is easy to work with, compared to the Grendel's 20-22". It why deer and hog hunters prefer it.
Aside from being an all out and very expensive showpiece, what will you actually shoot with it? That determines not only what barrel length, but what caliber. As for gas piston, it's unnecessary. It does nothing to add to reliability, only expense.