Gearing in general is very difficult to determine an overall acceptance level. As you may have noticed your compressors, even though they are identical, may have three quite different vibration levels and the one with the lowest level may in reality develop a problem before the others. The best method that I have found for identifing problems with these types of machinery is to trend on amplitude in units of acceleration. Changes should be investigated and narrow spectrums compared against baseline data. If the spectrum has a bunch of peaks with as opposed to one with only a few peaks, I will be more concerned with the complex spectrum more or less independent of the amplitude unless it is very low.
Gears are very complex animals from a vibration standpoint.