Caveat: I do not have the Insanity series (just one workout that came with Tony Horton's One-on-one series), but I have followed discussion on it with increasing interest, and I do have other workout series with "add-ons" that are similar in structure.
As Wendy said: don't think you have to do them all at once, or all within a one or two-week period. By spreading them out, you'll make the ' newness' factor last longer, avoiding boredom, and also not overdo.
I'd follow the basic rotation, keeping the add-ons aside (but preview them all, just to make sure there are no glitches) until I felt like doing something different (subbing an add-on workout for a similar workout in the basic rotation).