The gears in the diffs don't have anything to do with the drive ratio. They are just a means to transfer power from one side of the axle to the other.
Same here. I usually look at stock gearing and take a stab at it by adjusting tooth counts according to any modding I've done and test temps...
Yes, and with the Max8, you would have more room to grow, like maybe if you later decided to use it in a heavier rig. But as was mentioned, your drivetrain may not handle 4s, at least without doing some upgrading.
You have no need to go with a Max6. That thing is huge. A max8 would be more than enough, but as @SpareParts said, he is doing just fine with a Max10. If you are just going to stick with 3s, I'd probably just go with the Max10 as well. But I have no experience with the Revo's. SoareParts seems...
The number in the HW Max ESC represents the scale of the RC they are designed for. 8 for 1/8th scale, 10 for 10th, etc. But weight of the RC is going to have more of an impact on stressimg an ESC than the size of the RC. So if you're building a heavy 10th scale RC for example, you might opt for...