How do I set a slug validator?

You can control if a collection of slugs is valid or not by setting your own validator:
\Aero\Store\Routing\Slugs::setValidator(function ($slugs, $resolver) {
    if ($resolver !== 'listings') return true;
    if ($slugs->count() < 3 && $slugs->where('model_type', 'tag')->count()) return false;
    return true;
});