Upgrading my RAM in MacPro. Do cards need to be matching sizes?

Hey Everyone,

Just purchased a brand new Apple Mac Pro 12-Core Mid 2012

I purchased 4 - 8GB cards of RAM from Crucial:
2x 16GB kit (8GBx2) DDR3 PC3-10600 • CL=9 • Unbuffered • ECC • DDR3-1333 • 1.35V • 1024Meg x 72 • Part #: CT3309457

I received the computer with 6 - 2GB cards of RAM.
Size: 2 GB
Type: DDR3 ECC
Speed:1333 MHz
Status:OK
Manufacturer:0x80AD

My computer has 8 DIMM Memory Slots.

Is it wise to mix the 8GB and 2GB cards together?

Should I install my 4 - 8GB cards and leave 4 - 2GB cards or should I remove all of my 2GB cards from the computer?

Leaving all of the cards would obviously give the computer more memory, is this even necessary?

If I should give the computer the most memory possible, then is there a specific order the different sized cards should be placed into the DIMM slots?

The sales rep at the Apple store said it would be fine to mix them but I'd like a professional opinion.

Here are the specs to my Mac if needed:

Processor 2 x 2.4 GHz 6-Core Intel Xeon
Model Name: Mac Pro
Model Identifier: MacPro5,1
Processor Name: 6-Core Intel Xeon
Processor Speed: 2.4 GHz
Number of Processors: 2
Total Number of Cores: 12
L2 Cache (per Core): 256 KB
L3 Cache (per Processor): 12 MB
Memory: 12 GB
Processor Interconnect Speed: 5.86 GT/s
Boot ROM Version: MP51.007F.B03
SMC Version (system): 1.39f11
SMC Version (processor tray): 1.39f11

Thank you in advance for your help!