I've used the Meguiar's clay several times and never had any issues with it. I've heard most clay in the US comes from the same place anyway. However, you might want to buy a second bottle of quik detailer. As others have mentioned you need to really lube the surface and the supplied bottle of detailer was never enough for me to get the job done. I used about 1/4 of a second bottle.
Before you even do any of this make sure the car is washed properly. Do you know what the 2 bucket wash method is? If not look it up on Youtube. That will help immensely. You're also using Iron X which will remove allot of the crap the clay would have picked up in the first place so wise decision with that purchase. Keep the scrubbing to a minimum and don't introduce more contamination into the process.
Let me add, this is the first summer that I will try my hand at polishing after claying and my cars always looked decent but I always took my time washing and doing that part correctly. In the past I have always had silver or white (My current car, which is red, is my first car that shows the swirls more) cars so the swirls and marring were much less noticeable but of course they were there. Like you, I, didn't have the polishing tools but don't let that stop you from doing as much as you can to protect the paint. Your Mustang will come out decent once you Iron X, Clay and wax but as mentioned you will likely see some swirls and marring. That beats having no protection on the paint thought, right? Let us know how it works out for you.