First, I don’t think guides would be absolutely necessary if you’re not doing anything requiring any precision. Sand plotting, sketch drawings, etc should be fine “un-guided” I would think… although I would still want them to track as parallel as possible, to minimize the drift.
I have the motors on my tractors wired in series… ala MPCNC. I’ve never any sync or imbalance problems with them hooked up this way.
Not sure how you might use the rack and pinion idea (as “track” to guide/run on?) but the joints were not a problem at all. I have an Onshape rack design with ONE tooth… which you then “linear array” to the number of teeth you want/need. My printers can only handle a rack section a little over 8" long so that was the longest rack section I could do. On my R&P MPCNC I just staggered the joints on opposite sides and never noticed any issue at all when plotting rulers, crowns, laser engravings, etc.
I’m not sure the effect bigger diameter wheels would have. Playing with my machine as I have been, I have come to the conclusion that this is not the most stable of machines… when I gently wiggle the machine from the top of the Z-axis stage, the whole machine rocks, pretty much in any direction. Anywhere a “wheelbase” could be increased (tractors and linear stages) would help the cause… though it’s probably just fine “as is” if only light loads – pens/markers, needle cutters, lasers, etc. – are all it ever sees.
BTW you are welcome to any of my STL’s if you think they’d be of use to you. IIRC you once mentioned the extrusion “pass-thru” on the end-plates and the Z-axis linear stage… and I meant to offer them to you then but I forgot… 
– David