I just did a very quick and rough assembly... and well... it works kinda, the wheels without any plastidip or similar on a wooden floor slip and slide a ton and my tiny DC motors with not much down-gearing in the gearbox spin way too fast, the robot moves in the expected direction reasonably well, but it's basically spinning at least 4 times the distance is moves if that makes sense, and since it has 4 wheels with no suspension or give what so ever going over anything makes at least one wheel free-wheel in the air.
And this is without the 300g battery, motor controller boards, arduino mega and second level on top, I expect the performance to get even worse with that weight on top.
I'm re-spinning the design, going down to 3 wheels and switching to a "medium" sized servo with lower gearing for power on the inside of the wheel, and a bearing block on the outside for more support, the design is pretty much a triangle with a flattened corner and a wheel fairly close to each corner, wheels going through the triangle in holes just big enough, axles on the underside the 6mm plexi base, this way I can use a servo horn like you have for rotation, and an axle going through the wheel and into a bearing block on the outside for more support, axle doesn't have to marry at all to the servo, it's just there to support the wheel and hold it sideways.
I'll need to find something like plasti-dip to coat the rollers with as well, clearly needs more traction.
Quick sketch http://cl.ly/image/1x3y0A2u1X19