the question of using a 12 volt relay in a 6 volt circut.....clamping force for the output contact is you main concern to prevent early burn out due to light contact and possible overdraw of current and thermal runaway. So you have to make the call yourself. Here are some specs to guide you on the operational paramenter of the stock bosch mini relay (square one that re 30 and or 40 amp rated)
General Specifications
Bosch Part Number: 0332209150
Voltage: 12V
Current: 30A
SPDT
Enclosure: Standard Cover W/Bracket
Footprint: ISO 5 pin
Coil Specifications
Coil Operating Current: 160 mA
Equivalent Coil Resistance: 69 - 80 Ohms
Pull-in Voltage: 8 V
Drop-out Voltage: 1.2 - 5 V
Operating Conditions
Operating Temperature: -40 - 100 C
Humidity Test: 95% for 7 Days
Salt Spray Test: 144 Hours
Dust Test: 8 Days
Life Tests
Resitive Load - N/O: 30 A - 150,000 cycles
Resitive Load - N/C: 20 A - 100,000 cycles
Motor Load - N/O: 40 A - 150,000 cycles
Motor Load - N/C: 15 A - 75,000 cycles
Lamp Load - N/O: 30 A - 150,000 cycles
Lamp Load - N/C: 15 A - 50,000 cycles
Mechanical life: 1,000,000 cycles