Can you wear a 20g stud without any issues in a healed nose piercing that was pierced at an 18g?

You can indeed. A 20g stud is smaller (0.8mm) than an 18g stud (1.0mm) the only difficulty would be if you wore the 20g for a long time and then wished to go back to the 18g at which point the 18g would stretch your piercing slightly. However, even that would be such a small stretch that, while more difficult to push through, you should have no issues apart from some stinging.