The naming (which is also the location) of a sitemap is - contrary to what you state in your question - mentioned in the guidelines:
Also the guidlines clearly say that you can specify more than one sitemap file in
You can specify more than one Sitemap file per robots.txt file.
BTW Sitemap index files are used in case your file grows larger (in terms of entry count, e.g. 50.000 -or- in terms of file-size e.g. 10MB (uncompressed)) or in case you have multiple hostnames in the URIs which requires you to have multiple files - one per hostname.
BTW, the following answer is related on Stackoverflow, but you need to ignore the accepted answer as it is not well researched (like the question itself):