I don't know simpleacaptcha, so I can't comment on it.
But if it's not written for FuelPHP, chances are it will use standard PHP sessions to store the generated captcha. Which is a mechanism you can't use with FuelPHP.
I'm use mechanism database, all data in session is work, my project is in production at moment. This new functionality is in development. The package is write for FuelPHP... This project is in github...