I started out in psych and never intended to work in any other field, but the psych unit in which I worked was closed...in fact psych was eliminated totally from my hospital. So, I worked med/surg for awhile. I got a great orientation and felt comfortable being able to do the work (even though it wasn't my cup of tea and I left to go to another psych facility).
Sometimes psych treatment places are med/surg/psych, so med/surg skills come in very handy. Psych patients even in a non-med/surg/psych facility often have medical needs that make med/surg skills valuable. I didn't have those skills other than what I learned in nursing school
, when I was first working, so there were times I had to learn quickly, but it was definitely do-able.
As for becoming a psych NP...you will be doing lots of physical assessments at work, so it would definitely be a good thing to have experience in that sort of thing. You'll get lots of such things in your NP schooling though. LOTS!
Experience in med/surg will make things easier for you though.