I used a double rail on a couple of my earlier frames. Seems to me that I went with 7" to 8" center to center, equally spaced on either side of tube centerline. Not sure what is typical, use whatever works for you. A lot would depend on height of rail material and diameter(curvature) of raft tubes, especially if you are adding a walking surface. I would opt for 6" diamond plate as it would give more room for adjustment on the oar towers.
  As for oarlocks beyond tube edge, I don't see any problem. Just remember that the further out they go the more leverage you get but the less stroke length in the water and a couple of inches can make a big difference after a long day on flat water. Have fun.