[Al Skierkiewicz]

To help you in your search,
Center the PD in the robot, this will keep the wiring short. There is no significant advantage to moving to #4 from #6. If you keep it short, the difference is millivolts. #6 is .0005 ohms per foot and #4 is .0003 ohms per foot. #10 is much better than #12.
Uninsulated terminals are available from the same sources and cost less. Use the right crimper. Those meant for insulated terminals will not give you the same crimp force.
Breakers that have tripped will not degrade. If they are repeatedly abused over a long period (5-10 matches) the internal contacts will pit and raise the internal resistance. Under those conditions, replace them. However, if they are warm to begin with, they will trip at a lower current than at ambient temperature.
For a little more than the cost of the Battery Beak, you can get a West Mountain CBA IV USB analyzer. This will draw current for a much longer time and give you a display that you can save to disk, and overlay in the future. This allows you to track the health of the same battery over years. It also will match the curves used by the manufacturer and calculate amp hour ratings. It will show battery cells that do not match. The Beak is great to put in your pocket and know if the battery you put in the robot is charged.
Use the "wire foot" analogy. At 100 amps, 1 foot of wire has a voltage drop that is predictable. #6 will drop 0.05 volts/ft, #10 will drop 0.1 volts/ft and #12 will drop 0.2 volts/ft. The stall current of a single CIM motor is 131 amps under test at 12 volts. On a typical FRC robot this more like 116 amps. That is the current it will draw when starting and anytime you are applying full throttle and the robot is not moving. Yes, 6 CIM drives have the ability to draw over 600 amps from the battery. The internal resistance of the battery is 0.011 ohms/11 wire feet. So at 600 amps, that is 6.6 volts dropped in just the battery. Yes the main breaker can withstand that for short periods but not forever. Loose connections (bad crimps, bad solder jobs, loose battery terminals, loose PD connections, etc.) can amount to several wire feet of loss per connection and can also raise the temperature of the device they are attached to. A loose terminal on the main breaker can raise the internal temperature above 100 degrees.