I think this is what you're asking:
"I want to allow the index.php file to
be served normally, but all other
requests follow this rewrite..."
To keep the index file from being redirected, you need a RewriteCond before the rule:
RewriteCond $1 !index.php
RewriteRule ^([a-zA-Z_-]+).php$ http://www.domain.com/myfile.php?page=inc-$1.php [NC]
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