Using a fountain pen will require retraining your hand a little in order to use them properly. Unlike ballpoints or pencils, fountain pens only have proper tipping on one side of the nib, which means that you can't twist the pen even slightly while mid-stroke, or you'll lose contact with the paper. I had to take it very, very slowly when I picked up my Metropolitan a few years ago. Being much more deliberate about each movement is really important, which was something I wasn't ready for at the time.
While I did not follow everything on this page exactly, it gave me the basic concepts I needed to understand where I was going wrong, and how to correct it! Writing from the shoulder, as opposed to twitchy hand movements, was the best piece of advice to start with. When we write with ballpoints, we tend to limit ourselves to only a select grouping of muscles in and near the hand, which adds a lot of tension to each movement. The unintended side effect of this is that the pen or pencil twists around, and it's so subconscious that we hardly notice we're doing it. By holding our hand steady, and moving more with our upper arm, the pen moves far less, preventing the nib from skipping.
One thing I learned about fountain pens fairly early on is that it's about compromise - some pens don't work very well on certain papers or with certain inks, writing with one means adjusting the way you write, cleaning with relative frequency... there's a lot to do in order to get the most out of using them, and that's not something everyone is willing to do! Practice writing with your arm as I mentioned above (and be sure to visit the page for a more informative breakdown of it!) and see if you like it. If not, well, at least you learned before you invested in more expensive pens or inks.
Line widths are going to vary from manufacturer and country of origin, so one size isn't going to fit your needs across the board. Ink and paper choice also affect how thick the lines will be, too. It sounds like a Japanese Fine or Extra Fine will be what you're looking for, but that will still spread out considerably if you're writing on cheap copy paper. I have a Pilot E95s in Fine, which writes closer to Pilot's Medium nib size on cheap paper, but otherwise gives a quite thin line on nicer paper like Rhodia, Tomoe River, or Midori. If the Metropolitan you have is a Medium, it could end up looking much closer to a Broad on cheap paper!
The Just Meet 0.5 is a much thinner line than most fountain pens, so an EF will probably be the closest to what you're used to. Luckily, I have both a Just Meet 0.5 and a Pilot Fine nib here,
to give an idea of the size difference on quality paper! It's still about double the width, but Fine is as low as the Metropolitan and other lower-end Pilot models go.
