I would go with the 5/8. You can typically get it for 1.75/foot if you find a local aeration supplier who keeps it in stock, but 1/2 would be fine as well, especially if you are connecting the 20' on a seperate line as the other 3.

Can't say much about the rotary vane as I have never ran one. FYI, you can get a 1/2 to 3/4 HP dual piston pump from good dealers on Alibaba for about the same amount as that Rotary.

I have a similar pond as you, except 2.5 acres with max depth of 11'. I use a double at 11 and 5 singles at 7 near the 4-7' dropoff.

HTH