[XXShadowXX]

You will most likely be pushed around alot with your current design (this is from my personal experiance). But omni's mounted to and prependicular to the direction of movement could spell trouble.
Also you don't need to mount your wheel in a perfect square, if you mount what I'm guessing is your forward and rear omni futher back you will increase stabbility. Just make sure your wheel that are found on the same axis (left and right, forward, and, rear) are mounted so that they lie on the same circle. If your really good you should be able to make both of your circles have the same center point (and center point of robot), but good luck...