In my testing I have arrived at a comfortable conviction that there must be 3 arms and they must all be pistons. At 2.5 width, you cannot get a track past either the input or the output or the glyph of unification. For the middle arm, it needs to be able to reach the center of the glyph of unification, which means it needs to be at least length 2. However, since it also needs to output, it has to rotate around, which it cannot do at length 2 without leaving the 2.5 width area. so it has to be a piston. the logic is more or less the same for the other 2, however I'm somewhat less certain about the top waste arm. A setup with better geometry might be able to use a fixed length arm at the top, however it would require more track. As long as the extra track needed is 3 or less it would be worth switching to reduce cost. I don't think the puzzle lends itself to that kind of geometry given that, to output, you must grab the product from below and push it up. Essentially, a track that allows you to send your waste to a hex that a fixed arm could reach and manipulate would be out of the way of what you need. 
TL;DR to make the top arm a fixed arm instead of a piston, you would need to add extra tracks to both the top and middle arms, which would likely negate the 20g benefit of switching from piston to fixed.