Hello,
61.153 talks about the ATP requirements. Reading the regs, it looks like you may have to get a US commercial single or multi first. Not sure. You must also have an IR. I think you can get the IR without going through the full US training course.
Prior to getting your CPL, you'll need to convert your Australian certificate to a US cert. You'll only get PPL privileges though. Once that is complete, you can go on to seeing that you meet the requirements for the faa CPL and ATP.
All your flight time counts, none of the 50% stuff. If you like, you can post a picture of your logbook and we can have a look so we can tell you how your flight time will count under the faa system.
Lastly, you seem to be a Dash 8 FO. Forget about logging that PICUS stuff while you are acting as SIC. All your time in the right seat is logged as SIC, 61.51(e) notwithstanding.