Actually, no, you don’t need to enable Mirage on the Speed settings AND add a page rule.
A page rule should actually be called a “page exception” It defines the exception to the rule set elsewhere on Cloudflare dashboard.
Both Polish and Rocket Loader, as well as Mirage, should apply to the whole site when you switch them on. You can use Page Rule to switch any of them for specific pages or sections of your site. Or do the other way around, switch them off for the whole site and create page rules that enable them for specific areas of the site.