Sounds like you know what you're doing.
Just for clarity, what I meant by traction wheels was not-omni wheels. You could just use the same wheels as your center wheels.
In my opinion, unless all 6 wheels are traction (aka not omni) wheels, you don't need to drop the center. The purpose of dropping the center wheel is to shorten your wheel base. This way so you don't have to counteract the moments from the traction wheels on BOTH ends of the robot that are preventing your robot from turning. If you have omni wheels on the front, back, or both ends of the robot, that moment is no longer there.
I apologize if I'm overstepping any boundaries, simply trying to pass on my knowledge. I'm sure the team will be successful with whatever it decides. Looks great guys.