If you're building a 1/4 scale, I don't think a 4" poof ball is going to be the right size. 7"/4" >> 1/4... The raquet balls used in this years' VEX game are pretty good for a 1/3 scale game, as the balls are 2 1/4" in diameter.
Team 40 did just exactly this, and demo'd some of their robots at the BAE and UTC regionals. They had probably 6 or 7 different designs, including omni-drives, tread drives, swerve, etc... For balls, they found some of those "stress relief" balls that you squeeze to relieve, well, stress. They're about the same size as the raquet ball, but have the same "squishiness" factor as the poof balls. It was a VERY nice setup, and I'm hoping that we can build a copy of our '06 robot with VEX, just like we did with our '05 robot.
I'm a huge fan of VEX, and I think that it's participation can rival FRC in just a couple of years. The matches are as exciting, the quality of the robots are on par with FRC, and the kids can truly build 100% of it. If the low cost doesn't grab your attention, they could you please pass some of that extra cash our way?
BTW, there's a whitepaper somewhere here, showing the advantages of using VEX for prototyping and demonstrations. I think it fits very nicely with where I hope this thread will go.
VEX Whitepaper
BEN