100 RPM motor and a 1:7 ratio should be fine with good geometry. At least, it works for us.
If you need help with geometry, I’d recommend checking out this thread here.
Edit: A 1:21 would work as well, but it would be fairly slow with a 100RPM motor. You may be able to get away with 1:21 200RPM, I’ve seen it done before with good geometry.