Spur pinion combos?

[Ozmaxx]

I am about to order a 7xl, 9920 and UBEC from Mike and need to sort out what to do with my gear set up.
Do I stay 32p and use the 72,70 or 66 that I already have? If so would a 20 tooth pinion be too high?

I see the Strobe slipper using an Ofna 51 tooth spur, what pinion would be ok with the 7xl on 12 cells? And do I need slipper pads?

Obviously there are varying strength set ups for various applications.

Can someone suggest an appropriate set up for what I'll be running?