That depends on the Rally track that you use. The respective track README file (see e.g. nyc_taxis) documents the exposed track parameters. Usually the tracks do not allow to change the index names though.
In our own benchmarks we grant the benchmark user superuser privileges, mostly for the reason that the cluster is only created for benchmark purposes and immediately destroyed afterwards. What privileges are required depends - again - on the Rally track but usually the respective role needs to be able to create and delete the indices that are used in the benchmark, call stats APIs and bulk-index data.
From a security perspective a dedicated role and user makes sense. You can specify the credentials via the
--client-options parameter (see the docs) and specifically the examples there how to do that).
From a benchmarking methodology perspective you need to make sure that the only workload on that cluster is the benchmark to get representative results. See also our blog post Seven Tips for Better Elasticsearch Benchmarks for more tips about benchmark methodology.