[JHSmentor]

One thing that I can see from a flexibility standpoint - if the center wheels are direct driven on 8" then that would work fine as long as you are only driving those 2 wheels. If you then attempt to drive 2 or 4 of the other wheels at 6" diameter (in addition to the center wheels), then you will need to be careful with your sprocket ratios to get the speeds to match (the smaller wheels will need to rotate faster to cover the same distance). Not that it's a big deal but something that you need to be aware of.