I would consider a PI that you worked under, but who is a science-subject professor, a science LOR. Honestly, the requirements are typically guidelines. In general, you want the best LORs that you can get, as long as one of them is a professional scientist, and at least one of them can comment on your professional work ethic, etc., not just how nice of a kid you are or that you got an A in their class. I sent in four LORs, one from a volunteer coordinator, one from an MD (probably useless, but I have a lot of respect for the guy), and two from PIs who I also happened to take classes from. But I am certain that their letters didn't talk about my class performance. The relationship and how well they know you is what's important.