Here's what you need to have on a compositing reel. You need to prove you can do thew following things; use keyers with a green/bluescreen, generate travelling mattes with roto, be able to track and stabilize , remove wires and rigs and of course, create cool effects. (Make sure you show "before" as well as "after". I f all you have is your final comp, there's no way to see what the raw footage looked like before, so your work can't really be guaged.) These skills are the "bread and butter" of comping, no matter what software you use. For bonus, include colour correction, some motion graphics/flying logo stuff, and particles. You also need to show that you understand light and shadow,colour and composition - the basics that will prove you have a good eye, which is the most important thing. Don't worry about music so much...some folks turn off the audio when watching a reel. Use new tapes ( custom duration, if you can,) and be sure to include a list of every shot on the reel, with a description of what you did. ALWAYS credit other people who were involved in the shot. Only include your very best stuff, and put the best stuff first (VFX supervisors have notoriously short attention spans...) Don't worry about how long it is, just worry about displaying the skills I described above at the highest quality you can deliver.
( If your wondering, I taught compositing at Vancouver Film School, and have been comping as a professional for about 4 years, and have had "Demo Reel Watching Duty" many times before...)
Hope this helps!
jon a