I like your thinking with the 50l. If you can shoot that, you can shoot anything. You figured out something that took me a while: the lenses max aperture is just that, a physical relationship to the focal length. That relationship determines the depth of field but the light transmission also must factor in the quality and number of elements as well as the coatings.
Roger at lens rentals wrote a good article about it.
I learned SLR photography using a 50/1.4 mainly at 1.4 to make full use of available (usually indoor) light. I had to learn how to make the best of it, I could only afford ISO 100 film in my student days. Its great that you are taking the 50/1.2 as a challenge, it's also good for disciplined framing.
Now wide apertures are a creative choice instead of a technical necessity, and I try to keep ISO at 800 or below on my 6D. It's great to still get reasonable results beyond 800, especially for low res web output.