It is not the oxygen concentration that determines when hypoxic conditions may occur. It is oxygen partial pressure. At 10,000 feet altitude some people brcome hypoxic at oxygen concentration of 21%. I do not think you can carry animals in conditions (oxygen concentrations) that are necessary for fire suppression. Operators loose a lot of animals when the halon fire suppression system is discharged. Halon concentration is very low (3-4%) for fire suppresion.
For a true package transporter it may be worthwhile to have the cargo compartment unpressurized. This may not be good for some electrical or electronic equipment or for cargo that is temperature or altitude sensitive.