In order to use the developer preview of Android-L it's necessary to use Java 7. But will I be able to use Java 6 with the official release of Android-L?
NO -- if you are developing, use the latest supported environment (currently that is Android Studio 1.0 with Java 7 or Java 8 (oracle will drop support of Java 7 in April of next year).
Do not stay with outdated, unsupported versions of Java ... no need to add problems to your day.