Do you have the ratios handy? I’m somewhat concerned with the torque that’s going to be passing through the pinions (especially on the centre shaft). If the black 3/16" material is hardened steel, you’d be off to a good start. That centre pinion (20 teeth?) looks like it will die first, if anything has got to go, though.
Also, with thin hubless gears, you’ll need to consider how you want to mount them to the shaft. Lots of ways (e.g. keys, adhesives, welds, hubs, press fits, etc.) are possible, but it all depends on how you want to be able to disassemble it.
I’d consider leaving a little bit of clearance between the side plate and the face of the gears. In an ideal world, there would be exactly zero clearance, and exactly zero friction between them; in real life, the gear will wobble somewhat, and touch the plate, probably producing visible scratches, and yielding correspondingly increased friction (at least until it wears in).
Regarding the CIMs, am I overlooking a spacer somewhere, or have you constrained the front of the CIMs’ raised centre protrusion (you know, the 0.750" one with the shaft in the middle—what’s this called, anyway?) to the surface of the plate? You probably want the faceplate of the motor to sit flush instead, and to leave a hole for the centre protrusion.
Regarding the diameter of the shafting, you’ll probably need to use something torsionally rigid to use that size; almost certainly steel, and pre-hardened, precision-ground would be a bonus. And of course, I assume a nice set of ball bearings will be added in later…