I've read the Google docs on sitemap formats but there's one thing they don't make clear: will search engines automatically look for and find /sitemap_index.xml or do you have to tell them about it via /robots.txt or the main /sitemap.xml? Can you not have a /sitemap.xml and still rely on /sitemap_index.xml to be found and harvested?
In your robots.txt you can point to a sitemap or a sitemap index file using the same syntax:
Sitemap: <location>
Search engines will know by looking at the file what kind it is.
It is worth noting also that sitemap.xml and sitemap_index.xml are only suggested filenames, you may use any names you wish. Unlike the known location of robots.txt, search engines will not check any location such as sitemap.xml or sitemap_index.xml without you telling it that there is something at that location.