Re: replacing only 1 tire
I have done this twice, once at 1600 miles and again at 5500 miles. Both times they were rear tires. No noticeable difference in handling.
Only thing to make sure of is that each tire's pressure matches the one opposite. Front and Rear are expected to be different but Right and Left should be identical.
I had a brief period where they did not match and the effect on handling was significant even though the variance was minor.