Change idle RPM
 

Is it possible?

Certainly. On a carburated car there is usually an adjustment on the carburetor. On a fuel-injected car you're looking at getting a tuning module, which ain't cheap, if it's even available.

It should be possible with most any car. I have a 05 nissan frontier (obviously fuel injected and computer controlled) and there is a weird turn key, press gas 10times, turn key, stomp on brake peddle a few times, etc etc and it resets the idle to stock.
I found my idle creeped up about 100rpm above stock.
Also, if you want it lower that stock (which will probably make the car vibrate) the dealer CAN do it, but unless you have an 'in' I doubt you will find one that WILL do it.

My tbird and capri have adjustment screws on the throttle body. Depends on how old or new your fuel injection system is. Thats just for base idle though. You have to usually ground a certain wire to set it. After that computer and IAC take over.

My Tercel just has an easily accessible screw in the throttle body. You can raise and lower it however you like.

My Peugeot seems to have two idle speeds: one is when standing still (about 750rpm), and one when rolling (about 800-850rpm). Lowering the first may or may not cause vibrations, but lowering the secong to 750 should be OK?

I should say my method is with drive by wire... no adjustments on the throttle.