You can check out page 14 of the manual:

http://manuals.hobbico.com/sup/sup-manual-v1_1.pdf

The recommended break-in props are 12x7 or 12x8, and the manual says the recommended range is 11x8~11x9 or 12x7~12x8. Based on this information, I think the 12x6 will be fine for break-in. You might play around with a 12x8 or 13x6 if you want more power after you get a gallon or so of fuel through your G-75.