Although I prefer bold nibs, and the reasoning of your comment is sound (bold is going to be smoother and easier to push, which is what lefties do on paper), you can definitely get away easily with medium nibs too as a lefty, unless they are badly tuned.
Fine and extra fine sized nib are a matter of 1. taste in smoothness and line size, 2. how well tuned they are, and 3. hand writing position:
I don't enjoy particularly the fine line they leave, but I have at least an extra fine (european, on a Diplomat Aero) that writes really well, and it has very little feedback when sliding on paper, to the point it's almost smooth (I prefer smooth over feedbacky) even left handed.
There are cheaper pens on the market to try out different nib sizes, if one enjoys pens like I do, definitely find your favorite
And these are the most common nib options, there's even more :)
Unlike a rolling ball pen, it won't skip or fail to roll with pressure due to the rightward angle-of-attack against the paper, so a proper fountain pen is actually better than most other pens for us lefties. First time experiencing smooth writing in my life.
Ultimately, the enemy of lefties is the pointy tip, and the rolling ball that's expected to be rolled against the paper, not pushed against it. Felt-tip pens work just fine for us, so those pen Sharpies are alright, but so are the 1.0 bold tip roller ball pens from Pilot, at least all the Pilots I've tried. Can't say all the others work as well.