my suggestion is set up a WP install in another folder, or on another domain or subdomain and duplicate the conditions you will be working with. Just do a minimal install, with the default theme and a few posts and pages. Set your WP permalinks as you intend to have them.
Copy some of the HTML pages you intend to keep in that folder into the WP root. Then see what happens when you type their URL's in the browser address bar. You may have to set up redirect coding, for example in the .htaccess file, before the WordPress redirect code in the same file, to allow files with the extension .html and whatever else will be in there to not redirect over to WordPress. Because otherwise everything in the WP folder is automatically redirected to the WP index.php file.
You can also redirect them from within WP itself.