For fast vocals in general, what I tend to do is slow the song speed down in Reaper (it’s a slider in the middle of the screen, starts at 1.0). If you do this, make sure to set it to where the pitch stays the same when slowed down (right-clicked on the slider, and select “Preserve Pitch of Audio Items”). You want to set it back to normal afterwords, of course.

As for other instruments, a lot of time modern rap uses drum machines that tend to include really fast hi-hat patterns. Keep in mind that if these patterns also have snares over them, you may want to chart it as a disco beat. See the final chorus of “Day N Nite (Nightmare)” by Kid Cudi for an example of this.

A lot of modern rap, especially trap rap and a lot of things associated with it, also uses subbass, which if there’s no other bassline, would probably be best to chart to bass, for inclusion’s sake. This is, of course, assuming you’re doing full-band charts.

I believe so, yes, though I personally like to make them at least 1/32nd long when possible.

As for practice sections, they’re covered in the mirror of the RBN docs, here’s a page on them. The gist of it is, go into the Events track, double-click where you want to add a practice section in the grey area below where MIDI notes would go, load the Events text list, and select from the practice sections available. You should also set up things such as crowd animation and clapping, when the crowd initially cheers for the song (when a crowd would realize what song is playing would be ideal) (this is called “music_start”), and when you want the song to end in-game (called “end”) (the last two are required). Those are also in the same text list.