Hi,
If you run it on tracks anyway, two smaller angled ball bearings replacing each wheel should be good.
If the bearings you want are for the pan/tilt then a thrust bearing is needed for the pan (to carry the weight of what's on it), but I'm not sure where you can get it in 16", if at all available - they probably should be made to order (start saving up).
You could make a ring with small bearings used as wheels instead - like how it's done in many µWave ovens rotating plate.
However, I think you overdo the measures - balancing and a well placed amount of high viscosity silicone grease will go a long way to reduce oscillations.