How do I make every single NPC a certain character?

If you can't tell, every NPC in the sidewalk that's there is the same character, (Superintendent Chalmers) I'm also trying to recreate this video, but it's not for a mod or something. Please don't criticize me. (again)

Modify the pedestrian groups in your leveli.mfk script.
You can find the script in scripts/missions/level0* (where * is your level number)

But where in the script would I modify the NPC?

Scroll down for the pedestrian groups.
Code should be something like this:

CreatePedGroup( 0 );
AddPed( "male6", 2 );
AddPed( "girl4", 1 );
AddPed( "fem4", 2 );
AddPed( "boy3", 2 );
ClosePedGroup( );

Replace your models with whatever the modded chalmers model is

Okay. Also, are the numbers next to the quotation marks for the pedestrians are for how many there are?

//
//ADDING PED GROUP FOR LEVEL 01//
//
//z1r1
CreatePedGroup( 0 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
ClosePedGroup( );

//z2
CreatePedGroup( 1 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
ClosePedGroup( );

//R2
CreatePedGroup( 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
ClosePedGroup( );

//Z3
CreatePedGroup( 3 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
ClosePedGroup( );

//R3Z4
CreatePedGroup( 4 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
ClosePedGroup( );

//R6Z7
CreatePedGroup( 5 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
ClosePedGroup( );

//R7
CreatePedGroup( 6 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 1 );
AddPed( "chalme_m", 2 );
AddPed( "chalme_m", 2 );
ClosePedGroup( );

//Z6
CreatePedGroup( 7 );
AddPed( "chalme_m", 7 );
ClosePedGroup( );

Also, somehow this doesn't work.

That's because "chalme_m" is the name of the character file & not the character.
Open the file with the p3d editor & find out what the skeleton is called & use that.

I'd also suggest having one entry per group, with 7 as the weight - basically make them all like the last group

Ohhhhhh....

I'm also curious on how to make the custom siren and custom car music featured in the video.