Personally I think the whole thing is a bit of CYA from castle, and I would only really be concerned about it if I was pushing my system to the limit. What you are doing will not, and that just fine.
My first revo conversions used a MM on 4 and 5S and ran 10C batteries. they had no problems, but I didn't abuse them either. Decent batts and setup and it will be fine.
As far as gearing goes,
use this calc of BrianG's IDK all the ratios, but its quite easy to just use the drop downs to get what you want. FWIW, I used 3.5v/cell and 6.7" sized tires to get the speeds I was seeing on my radar gun using maxx mashers. YMMV, but it gets you close. Aim for 40 and a ~32K rpm motor and you will be golden.