Using plug-ins with ClassiPress is risky business in my opinion.
I've stopped development on CP due to it's oddity of nature - and so I am not fresh with it.
But to address your original problem;
My solution was to:
Add a Page with a "form" that the user must fill out to register.
Change the "Register" link so that it pointed to my register form page.
The form sends me the application and I manually register them.
Or;
I changed all the instances of
wp-login.php to something else.
Then provide a link (in the form email) to the newly named login file.
There are several ways to address your approach, but none are as simple as using a plugin. Sorry that I don't have a better solution.