XRAY - Model racing cars

Step #2, Central Transmission, Page 10, There is an important note about screw #5 that I'm a little confused about. I will be running on a new indoor carpet track which should be med-high grip. Does that mean I only install 1 - #5 screw in the motor mount plate front hole location shown and no other #5 screws are installed?

2nd question. I will be running in a 17.5 turn stock class, medium sized carpet track w/ at least one long straightaway and some technical section, rubber tires and no suacing allowed, 2S lipo. Any reccomendation on what size pinoin I should start out with using the kit included 84T spur gear?