I have a client site that runs on Wordpress, and of course the WP default .htaccess file always looks like this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

They have some old doorway pages some guy created long ago (I know - I hate them), and to work correctly they also need this code in the root .htaccess file:

RewriteEngine On
RewriteBase /
RewriteRule ^([^/]*)\.htm$ /page.php?str=$1 [L]

My question is (since I'm not a big mod_rewrite guy) - does this two mod_rewrite sections "conflict" at all? i.e., can they both co-exist in the same .htaccess file?

Does wordpress use any (fake) URL strings that include .htm/html?

The RewriteRule ^([^/]*)\.htm$ /page.php?str=$1 [L] is rewriting someold.htm pages to page.php?str=someold so that would only conflict if you're going to be using .htm* extensions for php pages and I don't think you are by default so it shouldn't be an issue.