we did exactly this, and i would recommend against it. 6 wheel drive with the outer wheels as omnis turns way to fast and is difficult to control without a lot of driver practice.
AM trick wheels are not what i would recommend, the plastic omnis are excellent in the 6 inch size, and they are also not quiet 6 inches, they are actually 5.94 if i remember right, which means you don't need to make your center wheel lower.
if you want to make a tunable robot, i would take a look ate 1986 team titanium's robot from this year, they used an 8 wheel tank drive, with omni wheels on the ends, and were amazing, because it still has 4 traction wheels, it doesnt turn too easy, but turns better than a standard tank drive.
you might try reducing your gear ratio some if you want to turn better, even just 5 percent will make all the difference in the world.
in summary;
if you want to use omni wheels on the ends, use 8 wheels total, 6 turns way to easy and its easy to get spun, and the plastic omni wheels are the best option as far as cheap, versitile, durrible, and lightweight.
Edit: here is a video showing 1986's drive train, if you want an drive with omni corners to learn from, theirs is very good.
http://www.youtube.com/watch?v=DdnYpgoClW4