If this is a single player mission, I'm fairly certain you can safely ignore that warning, but otherwise, find all the vehicles that you don't have actually moving and replace them with the parked version from the decorations. I don't think removing people will help.
Note: maximum number of people allowed in an SP map is 255 plus the player.
As for the .bin file, I know I've written up a thing about how I do them, but I for the life of me can't remember where I posted it.
Maybe this:
http://novahq.net/forum/showthread.php?t=46871