When I migrate my application to V7 using Vaadin 8, is it mandatory to use appComponents built under platform 7 and vaadin 8?
You cannot use application components with v7 that are compiled with v6. Version 7.0 is a major version with tons of changes and there is no binary compatibility between 6.x and 7.x release lines.
In order to migrate your application to v7 you need to migrate all your application components or wait until contributors will migrate them and make compatible with 7.0 (if you use community add-ons).
Migration efforts are not that big, but they are mandatory.