So I submit a 500 passengers run, but if i hit a better time on 100 passengers, can I submit the 500 passengers run as a 100 passenger run?

Yes, 100 passengers is a split of 500 passengers. Same can be applied in 1000 passengers. Unlock Completion is also possible but, the restrictions make most times slower. On the other hand, Overcrowd and Survival have different goals in mind. So not those two.