As far as I know you can use any kill card on your desk at anytime before a mission or to have some special effect on your career like gaining Merit or so.
After using the Kit Carson Scout Card he will be in the LZ every time you land at the beginning of a mission.
He will start a mission ALWAYS in the LZ so beware of those missions that you do not land there because he can be detected and thereby cause a mission fail.
Sadly once you have him the only way to not have him on every mission thereafter is to either kill him somehow or leave him in the AO after getting out.
Here an example vid of mine where he was very useful.https://youtu.be/Jw_tI7CReW4