If you use the better EMS mod you can do this. There is a configuration file that you can edit what type of vehicle responds with the ambulance. You can pick what kind of response you get and where all set by geography in the game. It's really easy to do. Downtown, I have a mix of EMS SUV Medics and Engine companies responding to my calls. Out in the Blaine County area I have an EMS squad that responds to some calls. You'll of course need to have those vehicles modded into your game though.