[Nonfractal]

Thank you for posting that. 

The hardest thing for me with gears is understanding how to select the correct pairing of gear diameters.

Any simple rules to follow...?

Brian, 
Sorry it's taken a while for me to get to replying to this...
There is no universal and easy to use reference for pair matching.?
If only... but I don't have the inclination to create such a monster. ?

However, here are some clues

1. Inline gearing diameters
The diameter of an inline crown has no effect on the gear mesh.

2. Anglewinder and Sidewinder
Start by referring to the manufacturer web page and make some calculations.
NSR anglewinder: the recommended crown and pinion is 16.8mm & 7.5mm. (16.8+7.5= 24.3mm )
Slot.IT anglewinder :  15mm & 7mm or 16mm & 6mm (= 22mm )
If you want to use a pod from one manufacturer but don't want to use their gears, you have to find a pinion and crown that result in the same total 
( +- 0.1mm )

Different manufacturers use different angles for their angle winder pods. (NSR 15°, Ninco 20° , slot.it 25°)
Match it if you can but it's nowhere near as important as hitting the right diameters.

Be aware that the boss on different manufacturers' crowns can be on opposite sides of the crown so ensure you select a gear with the same as the factory fit.

If you're dealing with a niche manufacturer's motor mount and cannot find the spec...
Good Luck! (You will need it)

I hope that is somewhat helpful ?

AlanW