Not a lawyer but go check out Wikipedia regarding
Fair Use. Assuming you're not slandering the brands in question, you should be fine. If you're in the US or Canada, I assume the definition of slander/libel is to spread falsehoods that damage a reputation. Here in Korea, unfortunately, if it damages a reputation, you can be sued even if it's true...
Parody is a tough choice, though, since Weird Al can get away with copying melodies and rhyme-schemes but Youtube-stars can't. In any case, avoid direct plagiarism.
Oh, and good luck.