First, there is NO reason to go from Mississauga, to Toronto, just to get back to Mississauga. It just makes no sense.
I don't know the Greyhound system, but I do know there is a Mississauga Transit bus that goes from Dixie Mall to Square One. From Square One:
http://www.cybertron.ca/boards/showthread.php?t=32120
If need be, check out
www.miway.ca
It's actually quite informative. I used it to figure out how to get from Mississauga to Etobicoke.
