I will let bobnh or another of our professional electricians address the specific issue of the codes on this.
I would make 2 comments..
1) you mentioned tapping off a light.....this sounds like a source controlled by a switch, and I assume the radon fan should run 24/7. So you may have to rethink your approach.
2) You mentioned K&T and other "mishmash". This can be a very confusing mess for a "weekend electrician" to jump into. It makes it much more likely that you might frankly screw something up. Maybe you need to call in a pro on this one.