Don't know for CS5, but I have both CC and CS6 installed on my computer, and it works very well.
If they are registered under the same name it should be ok.
Yes. I know a number of people who have CS4, CS5, CS6, and CC all running on the same machine.
I personally have both CS5 and CC on my home computer. (We don't have CC at work yet - grrrr).
Thank you guys for the answers !!