Hi Shah,
The only reason for 6 tubes is to save a little weight and save on making extra hardware you don't need (truss fixtures). As Cristian says it is just as stable as 8 tubes. Incidentally you may notice the piece of aluminium angle on the mirror box. By attaching the truss split blocks to it, I was able to do away with any shims - which can otherwise be difficult with six tubes.
I did originally have a single upper ring. It worked fine - no stress or tension. The way to avoid this is to close the triangle at the top. I use a modified version of Krieg and Berry's offset bracket design crudely based on this example:
http://www.wcc.net/%7Emyastronomy/AT...ss%20tubes.htm
The only problem with the single upper ring was that I became nervous about the truss tubes hitting the secondary mirror during assembly. It was also harder to store it in the car safely so I compromised with a very squat secondary cage which protects the optics better and can sit on top of the mirror box for transport.
I agree Bruce Mills' scope looks great. It's way beyond my skill level though. You would need to add light baffles - Mel Bartels has some excellent advice on this. A baffle just below the focuser is very useful on a single ring as is a baffle on the ring opposite the focuser. Under the focuser I use a camera iris I bought from surplus shed. It is helpful to also have some plastic or black fabric cover the back of the mirror cell to stop any light being reflected up through the the bottom of the tube.
If you build flex rocker, try to minimise the size of the base ring if you want to build an equatorial table. The base ring on mine is over 20 inches across, so an equatorial table to suit it would be very large. Consequently I am going down the Bartels stepper drive route - more work but it is an interesting project and there is lots of help available on the net.
Sorry for the long ramble, hope some of it's helpful,
Rod.