I was under the idea that I needed FI and CPL to be able to be paid for FI work, I guess that's wrong?
You can be a FI with a PPL but to teach other than LAPL as a FI you must have passed CPL level theory, for the purpose of instructing there is no expiry date with this. You only need a Class 2 medical if you instruct as a PPL holder however; to undergo the FI course as a PPL holder you need 150 hours PIC compared to 100 hours PIC for a CPL holder. A PPL FI can be remunerated for Instructing and Examining.