We pivot the traction wheels down for a few reasons. First, by putting them nearer to the center of the machine, it makes turning with four high-traction wheels a bit easier to accomplish. We also gear down further to that wheel set, so it makes sense for it to be at the end of the gear train.
I was always under the impression that the traction wheels were suppose to be on the outside to aid in pushing/ not being pushed; that not being able to turn was desired because you are only ever going straight when pushing? Also, if the mecanums are on the inside won't you rock up onto the mecanums when pushing someone? Additionally, are you concerned with your wheel pods being bent in when your traction wheels are down and someone T-bones you going 14+fps?
The gear ratios are 8.45:1 at the mecanum wheels -- that's one of the options for the Toughbox Nano -- and about 25:1 at the traction wheels. The traction wheel ratio makes things pretty slow, but it's easy to swap in sprockets to speed things up as required.
Any concerns about the gearbox ripping itself apart when you are switching wheels at full speed and both wheels are momentarily touching the ground or do the mecanums slip enough for this to not be a problem?
The kit frame is 1.25" thick and the output of Toughbox is 1.5" from its edge, so when using a 6" mecanum wheel, we'd end up with just .25" of ground clearance if we built things the other way around.
How high do the inner wheels pivot off of the ground, and how dropped are the traction wheels when they are down?
The subframe, right now, is 1x1x.125" angle bolted to the perimeter of a .125" perforated PVC sheet and hung from the Toughbox Nano housings. It is intended to hold electronics and would need to be enhanced if it were going to see more substantial loading.
This is based on AndyMark 6" mecanum wheels --
http://www.andymark.com/product-p/am-0136.htm
We used this last season and I'm happy with the build quality and performance. They're heavy, but they're not as heavy as the 8" set we originally played with years ago and we're willing to deal with the weight and cost in place of building our own.
Our mecanum implementation worked. I didn't program it, so I can't speak too much to what made it work, but our programming team did a fantastic job there. We had accurate, fast, field-oriented drive that allowed us to fully use the movement capabilities of the mecanum wheel set. In my experience, most teams fail to achieve the level of control we managed.