I have a lot of mRSS (media Rss) files. I plan to create sitemap index to contain them.
Are sitemaps allowed to contain mRss files? To my knowledge, they are only allowed to contain image, video, and content in sitemap index. It is right?
Yes, sitemap index files can refer to mRSS files, but you can also submit them individually if you want. You can't link to mRSS files from normal (non-index) sitemap files though. In addition to the help center page you mentioned, there's also a bit about the differences between these files in an older blog post.