Since you have non-separated mixed use, whether or not the garage design is required to be per NFPA 13 or NFPA 13R depends on whether the garage is accessible by people from more than one dwelling unit.
From the '02 edition of NFPA 13R:
6.7.3.2 Garages that are accessible by people from more than one dwelling unit, and are not covered by 6.7.3.1, shall be considered part of the building and shall be protected in accordance with 6.7.2. (6.7.2 requires design per NFPA 13) Garage doors shall not be considered obstructions and shall be permitted to be ignored for placement and calculation of sprinklers.
6.7.3.3* Garages that are only accessible from a single dwelling unit shall be considered as part of that dwelling unit. Such garages shall be sprinklered with residential sprinklers in accordance with 6.7.1 or quick-response sprinklers designed to provide a density of 0.05 gpm/ft2 (2.04 mm/min) over the area of the garage, but not to exceed four sprinklers. Garage doors shall not be considered obstructions and shall be permitted to be ignored for placement and calculation of sprinklers.
So if you have garages that are dedicated each to a different unit, you can use NFPA 13R, but if you have a garage that is accessible by more than one dwelling unit, the design discharge has to be in accordance with NFPA 13.
(My question is, how does the fire know how many units have access to the garage??) ;-)