In lieu of advice on meeting like minded solo travellers, I'm just going to say this: Hi! I've been in Oz since the end of July, backpacking up the east coast, and am currently job hunting in Cairns.
If you dive with ops that also offer accommodation, you're bound to meet like minded people, although they may not all be solo travellers. The shop that WetPup mentioned in Alva Beach is one of those places... As for diving the Yongala, I had a couple of really nice dives there. It's definitely worth doing, but I'd try to get some more dives in first. When I was there, conditions were great, calm on the surface, no current, and decent viz, but if conditions aren't ideal, I can definitely see this as an intermediate dive, especially if you have little experience beyond 18m.