Unlike radio stations, I don't think there's any specific rules for just dropping *.mp3's into the \music folders, other than making sure you don't use anything too big (filesize wise - as that will cause a little lag while it loads). I think the biggest custom file I have in there is a 14 minutes s.t.a.l.k.e.r.
dirge :blink: - the rest are like 3-4 minutes (3-4 mb).
Make a copy of your Music folder before you start messing with it though - that way you can revert back to default music, should things go awry.