most servers are configured to read .html pages before .php, so it should go to the html page first (if you go straight to the base URL/domain)
I was running 60 wordpress site on shared hosting something went wrong and even when i dropped it down to 15 WP sites the servers were still overloading. Look at my sig.
Even one wordpress site can bring down a shared hosting account. Wordpress does a lot of stuff in the background as it is. When you load it up with plugins and start adding posts all day long you can overload a shared server pretty quick.
Yes, it wont rule. The index.php of the Wordpress CMS will still be active than the index.html unless you specially modify the .htaccess to read the index.html first.