Hope you dont mind the ole cut and paste...
1) The frame uses 700c rear wheel, so I will likely use the Spinergy Rev-X that I use now. If I pair that with a double chainring crankset 39/52, what cassette cluster do you recommend? It's currently set up as 13-23 8 speed.
Is it hill in your part of the world? If so your going to want lower gearing for the hills, theres no standing on a bent. Ya gota spin up them. You could really tear up your knees trying to mussle a bent uphill.
2) With USS, should I use GripShift or bar ends?
Use what you like, both work well.
3) Another Question about the 700c rear wheel: what size tire is recommended? Is the 23 I use now too narrow?
Keep in mind that you can't un weight the rear wheel like you can on a standard bike, I like to run larger tires on my bents to prevent pinch flats. If your light you should be able to use the 23mm tire.
4) Anyone use the Aerospkoe 20" front wheel? What is a good, fast, smooth 20" tire? Have not used the Aerospoke 20" on a bent, I have used there 26" off road wheels on a off road tandem. Good wheels, but a little heavy. I have use a set of HED wheels on my Longbikes Slipstream, nice wheels but usless for touring. The Primo Comet tire is a good smaller tire, with a "fishbone tread"
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle