In general, I wouldn't recommend it for one practical reason (unless you know them VERY well).
Nursing is a very different field from medicine with very different values. It is not atypical for nurses and nurse practitioners to value task-oriented ideas around evaluation, and a particular flavor of candor that is valuable for an employer but not necessarily a medical school.
If you do a search, you'll find students who ask their longtime NP colleagues for letters, whom later admit they talked about how the student was late, slightly off-kilter, or could not perform a specific task or procedure precisely because they wanted the recommendation to be "realistic" or otherwise credible.
For a letter no school asks for, that is awful information to volunteer about yourself, irrespective of whether it is actually true or fair.
I don't think it is necessarily deliberately malicious, but it is easy to see it that way considering what you have at stake. Stick to physicians and professors.