I used the flush and fill type kits for many years. I also installed the valve in the heater hose. You still will be opening the petcock for adequate draining, but you can get a better flush with the kit.
I would flush the system until it runs consistently clear, then drain, and add the right mix of anti freeze for your area. I imagine it would be a strong concentration where you are!
You can add a rust inhibitor, but I think good flushing and anti freeze renewal should do the trick for you.
You might want to check your thermostat while you are at it, as it might have been effected by the rusty water.
and scan your hoses also for cracking, etc.