Yes, you can set ISO 320 on an 80D. All Canon DSLRs (except the Rebel series) have 1/3 stop ISO selection that makes it possible to select ISOs like 160 320 640 etc.
You are correct. But, I just noticed that in P, Av, and Tv modes my T5 allows me dial in AEC in 1/3 stop increments.
Yes the entry level Rebels will let you select 1/3 stop apertures and shutter speeds and Exposure Compensation but for some reason you can only select ISO in full stops of 100 200 400 800 etc.
But.... If you use Auto ISO, the camera will sometimes use 1/3 stops for ISO.
So I am thinking about getting an 80D can it use the same lens, batts and flash as my T6i?
It can use all of the same lenses, but not the same battery. The Rebel batteries are too small for extended use with most of the bigger EF mount or “L” lenses. The xxD line of mid-range cameras, like the 80D, share the same battery with the 5D, 6D, and 7D series of cameras.