Intermediate is going to give you way too much dwell time, going to tear up brass with a 6.5 Creed with normal burn rate powders used with 6.5 Creed.
Rifle or +1" gas system may well have more port erosion but isn't going to be any harder on the brass then trying to extract case while chamber pressure is higher.
Definitely a pick your poison situation, I'd live with the port erosion and keep adjusting the gas block to compensate for port getting larger. Increased dwell time from an intermediate gas system, just no good way to countering that IMHO.
18" would be as short as I'd go personally and a 20" barrel with right contour isn't going to weight enough more than a 16.5" barrel for me to worry about. I see appeal to the 16.5" barreled 6.5 Creed AR but I do think performance of a 6.5 Creed in an AR below 20", even more so below 18", is going to take a much bigger hit then one would see in a bolt rifle. AR has more dynamics that come into play, 16-18" barreled bolt rifle in 6.5 Creed has far fewer variables and will most probably not lose quite as much performance.
16.5" barreled 6.5 Creedmoor in an AR you'll be about 2650 fps with a 130 AR hybrid. 600 yards that is 93" of drop, 26" of drift and 926 ft lbs of energy.
24" barreled 308 in an AR you'll be about 2550 fps with a 185 Juggernaut. 600 yards that is 102" of drop, 28" of drift and 1201 ft lbs of energy.
16.5" barreled AR in 6.5 Creed does beat out a 24" barreled 308 in drop (9" less with 6.5 Creed) and drift (2" less with 6.5 Creed) but not energy (275 more ft lbs with 308) delivered on target.
Shorter barreled AR in 6.5 Creed is not a shorter barreled bolt rifle in 6.5 Creed. ARs are not bolt rifles, you will not get the same velocity from an AR as you can get from a bolt rifle.
We need experimenters, if AK4900PA wants a 16.5" barreled 6.5 Creed with a rifle or +1" gas system I don't see it being a "bad" thing. Okay, has faster gas port erosion....so what? Adjust gas down with gas block or heavy up buffer some more or put on a new barrel. Barrels are cheap, life is short and if AK4900PA has the $, time and desire to do a 16.5" barreled 6.5 Creedmoor with a rifle or +1" gas system then he should give it a whirl.