This year, my team used a modified Octanum Drive that we called the "Dragonfly." It also used 6" AndyMark HD Mecanums mounted on the outside of the modules with 4" x2" Colsons on the inside of each module. We had no issues with being passed onto our medallion wheels, and with a Vex Pro Ball Shifter with a CIM and a Mini-CIM driving each module,we had more than enough power to spare.
The thing that worries me about your concept is the placement of the pistons that actuate the modules. You could easily bend the shaft or just have one side of your robot on mecanum and the other using traction just from another not hitting you hard and pushing. We had one piston actuating each module with both the pistons on one side mounted to a machined aluminum block.
Our not was an expensive, power hungry monster, but it did its job, and it did it WELL.
Here is a link to the CAD files.
And
here is a picture of our competition bot! Good luck!
And, just for the record, dlavery is on my mental ignore list from September until January if the thread involves the word "hint" at all. - Andrew Schreiber [more] 
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