I just returned from a week in Perth. I used the Perth Connect shuttle service from the airport to the CBD. Here is the website:
http://www.perthairportconnect.com.au/pac/index.php
The timetable shows times that would fit your arrival and your departure times.
Your itinerary sounds fine. I also did a day trip to see Wave Rock. It was about a 3 or 4 hour drive (one way) to get to Hyden, and I enjoyed seeing the rock.