Being interested in multiple PI's shouldn't be an issue at all, unless the individual program indicates otherwise. Some programs encourage applicants to list multiple faculty members, especially on the application. I would just make sure that if you're going to be listing more than one person, that you are able to explain your rationale for doing so. Additionally, when indicating interest in multiple PIs, it might make sense to list your top choice first in both your application (where applicable) and your personal statement. Order doesn't always matter, but it might occasionally.