@gonnif - Wow, thank you for the extremely detailed answer. I greatly appreciate it.
You said that the letter from my PI wouldn't count if the school requires a "science professor" though it would count as "science faculty LOR." If my PI is a professor at the school, gives me units for the work in the lab, and grades me for my work in the lab, can I argue that I've taken his "class" and label it as a science professor letter of rec? Or would that be stretching too much?
Sorry if that's a stupid argument! I don't know how many schools will specifically ask for a science PROFESSOR letter, so I'm just trying to know my options to the full extent.