If you're going with Greyhound, its cheaper to book in advance for their coach trip i think, though they are always regarded as the more expensive option. Ive never used a hop-on hop-off coach (my WHV adventure doesn't start till September) but i get the impression they are really more just a coach service, not a tour as such, so i wouldn't necessarily expect to make friends. Though as MARYROSE02 says, you'l be making friends in the hostels in no time so i wouldn't worry about that.
Just so you know, it'l be getting real hot and wet in Cairns from October. I'm starting in Brisbane and heading down with the Summer season, heading back up with the Winter. Not saying your way is wrong, just worth bearing in mind.