Brian: just use Bootstrap. It's easily the most well supported, and you CAN just use the parts you actually want, without the theming. If you just want the grid system, for instance, you can get a custom build with just the grid system. Also, the "looks like bootstrap" thing is overblown, sites only have the bootstrap feel because people are too lazy to make their own CSS. Bootstrap's theme is really good out of the box so most of the time folks just use it.
Also, you want a grid system with cutoff points because it makes your layouts way easier on the eyes. Sure, you can make a 100% fit page, but it looks AWFUL on larger monitors. No one wants to read a paragraph that spans a single line. By using a grid system, you use breakpoints to determine how to fill the available space. So if it's a large monitor, you might put the username box next to the post, and limit the width of the page, so that text wraps naturally. On a smaller monitor, you might maintain the horizontal view, but fill the available space. On a phone screen, you might put it on the top of the post, and also use larger buttons for tapping. That's the kind of power Bootstrap gives you and it's stupid-easy to use. Bootstrap 3's breakpoints are a little old fashioned, but the upcoming Bootstrap 4 (which is either in late alpha or early beta now) has improved them even more.
Overall it gives you way more flexibility, as you can decide on a per-element basis where things should go at what sizes. Making a separate CSS doc for every type is obnoxious and you'll either end up with a worse result or just reproduce what Bootstrap already gives you.