if I were to pick just one song out of the playlist, is there any way to get it to repeat?

As Muzza pointed out, there is a Repeat button on the Rio remote, and a Repeat option on the main menu. However, this is only available in version 2.0 of the software. If you are running version 1.02 or 1.03, then the repeat options don't show.

Is there any way to select a few songs out of a playlist and only listen to them? Then would there be a way to repeat these songs?

Yes, use either the playlists menu or the Search feature of the remote to find the songs. Select a single song if desired (replace) or select a single song and add more songs to it using Insert and/or Append.

Note that the following features assume that you're using the 2.0 software...

For how to select Replace/Insert/Append from the Playlists menu, see here. To select Replace/Insert/Append from within the Search screen, press the SELECT MODE button on the Rio Remote.

By the way. Sometimes, on the playlists menu, you will see a Musical Note icon. This icon appears if there are both songs and sub-playlists in the same menu. Select that musical note icon to see the loose songs.