You don't know how six rotors will do, since you can't see the site, or know the water supply. I can't either. At some point, the homeowner either has some faith in the installer's ability, or doesn't. I do know of plenty of installs on rectangles like the one described, with standard rotors, that only have six heads, and with adequate operating pressure, there are no dry areas. If anyone were to 'inform' the homeowner that the design is inadequate, and more pipe and heads must be installed, the homeowner would never go for it, since the lawn is already one hundred percent covered. There are other layouts for a 50x80 rectangle that would work on lower pressures, that would still require less than a dozen heads.
Head-to-head spacing is fine, but to insist it is mandatory is simply not living in the real world.