I wanna do all races runs of this game but I am not sure of the rules. For instance when the timer starts/stops, opponent count/difficulty, and lap count.

Some answers to your questions:

1. You should start the timer just before entering the first race.
2. The "All Races" category is for all Festival races, same with "All Gold". So, you don't need to set opponents or lap count. That's all. Please keep in mind that i am NOT a mod. If you want confirmation, ask Seluxity.

The timer starts when a new career game starts. For All races, as the name suggests, you need to pass all the races from the list for all ranks, not necessarily for gold. In the usual Any%, you do not need pass everything.

The category rules for each category have been added. So now you will know the rules to each and every full game category.