New Grad

Contrary to popular belief, you don't have to work in med-surg first to have a successful psych nursing career. It can't hurt, but it's not required. You will learn the same first year basics of prioritization, delegation, time management, etc., no matter where you start off. You will still (or should!) use your physical assessment skills because your psych patients will have med issues. And if you don't like m/s, a year in it will make you miserable.

As far as it being career sabotage...again, a lot of nurses have successful psych careers without ever doing a lick of m/s. If you do want to keep your options maximized though, a year of m/s will help you switch to other specialities...but mostly in the first few years. It will be of less help in switching down the road because many facilities are now asking for recent experience (as in the last 3 years). What you could do is get a job in psych and do m/s per-diem, or work psych-medical, and that will give you current experience in both specialties.