I know that a lot of folks would disagree with me, but as a psych nurse of some 20 years, I would say yes
-- not that you have
to, but that it would be a good idea. Psych patients have medical problems, too -- and even more so nowadays, as only the sickest patients (with multi-system problems) get admitted to inpatient units. In fact, med-psych units (for patients with both medical and psychiatric dxs) are a growing trend within hospitals. Even in outpatient settings, psych people have to deal with lots of comorbidity.
Also, one of the scariest things about working psych has always been, for me, how many staff (doctors as well as nurses) on psych units are so focused on the psychiatric issues that they completely overlook acute medical problems that are staring them in the face. I'll never forget the poor lady on our psych unit where I had my first psych job -- she was a chronic schizophrenic and v. familiar to all the staff on our unit; for several days she didn't want to get up and participate in activities, and complained of abd. pain. Well, we all charted faithfully that she was "seclusive to her room" and "preoccupied with somatic complaints" until she finally passed out on the floor, and it turned out that her inflamed appendix had ruptured a few days earlier, and she now had a belly full of flaming peritonitis! She had a long, difficult recovery following her emergency surgery ... I've never been cavalier about psych patients w/physical complaints since ...
i've seen more psych nurses do more "dim-bulb" things with patients than you would believe because they're so rusty on basic med-surg nursing -- like the experienced psych nurse I worked with at a major teaching hospital who brought a patient to the nurses' station for VS, checked the board and saw that the doctor had ordered BPs "sitting and standing" (I guess they couldn't deal with the term "orthostatic"), and said to the patient, "Well, since you're already standing, I'll do that one first ..."
You're free, of course, to do whatever you want, and you could probably find someone who will hire you into psych without any additional experience. But, esp. since your previous experience was in a specialty/outpatient area and you've not been in nursing at all
for the last 20 years, and since you asked for opinions/advice, I believe it would be a really good idea
to get some real med-surg experience.
I heartily recommend psych nursing, though! Best wishes with your new career!!