Looks a lot like our gearboxes from this year for our mecanum drive. haha. Spur gears were 20.5PA 24Pitch and so they're hiding behind.
Are you using big cim to save money on gears? If it's for weight, it wouldn't really help. The weight for a gearbox using a small cim motor and same final speeds will end up probably in the same ball park. But using two big cims in the whole drive, heaviest motors nice and down low, would allow the small cims to be used elsewhere up higher. But running at equal speeds, the small cim is more powerful than the big cim. I can't find the spreadsheet I used, but I remember that it was a decent loss, around 20-30 ftlbs, although when it really came down to it, it was traction limited. How much torque will the gearboxes be outputting before stall?