Coding in parallel of MidnightCMS

You can develop your own symfony application in parallel of MidnightCMS without any problems. You just need to configure the router to make sure that your route is placed before the "vfou_cms_main" route. If you just want to add a custom specific page to the CMS, you can just go read how to create a new page type. You can put your Page type file into the AppBundle or your custom bundle at will.