Internal Ratio is 2.055 i think. so its Spur Gear / Pinion Gear * 2.055. That is your FDR.
1) Don't know much about 8.5 and 5.5t. I race with classes in Toronto, so GT1 is 13.5T and GT2 is 21.5T. I do have a 9.5 T and I run at around 6?
2)Yes, you will need to , that is your Pinion Gear.
your FDR is (70/20)*2.055, where 2.055 is ta05 v2's internal ratio. You will really have to test it out. The lower FDR it is, more workload on your motor and ESC, hence creating heat.