We have Mecanum's and they have to be in an X looking down from the top or you loose maneuverability (I think either zero turn radius or strafing). Also, keep in mind that mecanum's have very little pushing power so if you plan to use these wheels your drivers better practice so that they don't get pushed and pinned.
Conceptually, you can think of mecanums as having two vectors, here is a very good PDF on drive systems
http://www.firstroboticscanada.org/s...irectional.pdf