You put a condition:
couatl.condition = %(numPassengers)d > 5 and %(numPassengers)d <= 20
Likely because you copied the [fltsim.1] from the Paintkit sample sim.cfg, which is related to the FSDT Shuttle, not the bus, which is the [fltsim.0] section, which is the one for the Bus.
This means your red bus will appear only if the estimated number of passengers is between 6 and 20. A couatl.condition inside a [flightsim] section will always take precedence to the a condition in the [general] section, which says the number of passengers should be more than 20.
For the same reason, the wheels distance is also wrong, because it was the one for the shuttle. Basically, you should have taken the [fltsim.0] section, which is for a bus, as a template.