RE: PAPER SERVO WIRE TUBES
The string method works VERY WELL. When you stand the wing ion end, try to align the holes so they are close to plumb and the nut will fall right through without any issues. No need for tubes in this case. In fact, I wouldn't even recommend them.....
As for the size of the holes, without getty too crazy, go ahead and make them a convenient size. Ribs provide very little strength. Ribs are mostly there just for shape. As long as you are not cutting holes directly between upper and lower spars, you shouldn't have a problem.