在应用程序中混合使用MySQL和Mongodb

I'm writing a web application using PHP/Symfony2/Doctrine2 and just finishing up the design of the Database. We have to import these objects (for ex. Projects, Vendors) into our database that come from different customers with variety of fields. Some customers have 2 fields in the project object and some have 20. So I was thinking about implementing them in MongoDB since it seems like a good use for it. I'm writing a web application using PHP/Symfony