Gear pitch

[asid61]

I was looking at creating a custom planetary gearbox using off-the-shelf parts (no CNC) and I came across something interesting: There are no 20 pitch ring gears. It goes from 16 pitch to 24 pitch and skips the 20 pitch mark.
16 pitch gears are too large to get any appreciable ratios, so I was wondering what people thought about 24p 20* PA gears for drivetrain? The gears come in 2024 aluminum, 303 stainless steel, brass, or some kind of carbon steel (not sure which). Because of the design of planetaries, this means the whole gearbox would be made of steel/aluminum/brass 24p gears.

Also, I was wondering what pitch/module gears were used in the versaplanetary so I could approximate load ratings.