Are you looking for "best practice," or trying to squeak by with the sloppiest processing you can manage that will still deliver an acceptable-to-you result?
If the former, then it is pretty clear that reusing fixer that has fixed film in it already is bad for fiber-base prints. As I understand it, there are by-products in film fixer that are difficult to remove from the paper, even with thorough washing. Plus, film fixer will tolerate a much higher level of dissolved silver and still fix film well than fixer for fiber-base paper. It is really a good idea to use a separate fix for fiber-base paper and to watch throughput capacity carefully. Careful control of your processing and capacities is pretty important if you are processing for optimal permanence. If you don't care how long your prints last, then, whatever...
I imagine that you might be able to fix film adequately in used paper fixer after it has reached it's capacity, but, personally, I'd never bother.
RC paper is a different story and I'm not familiar enough with the recommendations for it to comment on whether a single batch of fix would work for both it and film. Still, you'd be throwing off any kind of capacity control based on throughput, since who knows what the effects would be on the fixer capacity. I guess one could do extensive tests and come up with a factor, e.g., a "sheets of 8x10 paper" equivalent for X amount of film, but that kind of defeats the purpose of being lazy and using one fix for everything, doesn't it.
C-41 is also a different story; I'll leave that to the color printers to discuss.
Best,
Doremus