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Most guiding software does a pretty good job of measuring the guide star centroid to a small fraction of a pixel, so it's fine to have a guide image scale that's less than your imaging scale. The benefit of an OAG is that it sees through the main scope optics so flexure is eliminated, or at least greatly reduced, and mirror movement (assuming it's not a refractor) affects both guide and imaging fields and guiding can compensate for it.
At short focal lengths, especially with refractors, a guide scope should be fine so long as some care is taken with the mechanical linkage between guide scope and main scope.
Cheers,
Rick.
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