Which schmidt plow? If it is a 3 or 4 section plow, then I would say no, you can't narrow it unless you remove the outer sections which would leave you with a very narrow plow... the 3.1, 3.4, 5.4, ect all have a lot of engineering that goes into how each moldboard mounts to the support section so that it will individually spring away from obstacles, and 1 foot would get into the places where the support bars and springs mount. If it is a single piece plow, then it should be no problem to narrow up, if you have decent cutting skills and make sure that you cut where there is support behind the very end of the moldboard.
Cheers,
Ben