Opposing gears don't have the same form unless they have the same number of teeth. This isn't obvious when you look at the first gear pair, but look at the pair doing the 10:1 reduction. The teeth of the small gear are undercut quite a bit while the teeth on the big gear aren't undercut at all.
The only real constraint is that the teeth must roll into one another as you concluded.
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Opposing gears don't have the same form unless they have the same number of teeth. This isn't obvious when you look at the first gear pair, but look at the pair doing the 10:1 reduction. The teeth of the small gear are undercut quite a bit while the teeth on the big gear aren't undercut at all.
The only real constraint is that the teeth must roll into one another as you concluded.