How often is Shift, Control, Alt or Command combined with the secondary click? One option I have for activating expose is to combined Shift, Control, Alt or Command with a secondary click. I am wondering if any of these combinations are likely to interfere with any useful commands on the Mac. Do any of these combinations already do anything useful?
Interference depends entirely on the application you are in when you use that combination of keys. Based on my experience, the only applications that get into that deep of a combination of keys (shift+control+cmd+[...]) are very robust like those found in Microsoft Office and the Adobe Creative Suite.