Is a ROMs data (/data) from Nougat compatible with MM? I have a Nexus 6p with Resurrection Remix, Android 7.1.1. I want to move back to Marshmallow (still RR) for Xposed. I plan to just dirty flash the old RR 6.0.1 .zip file. Will this result in any problems with my data? Like my installed apps, data for those apps...?

Thanks in advance. If you need more information just ask!
From one of my earlier answers:


  Older version of an app might not recognize the data created by the newer version of the same app. Although user apps are rarely affected, crucial system apps might be, and on error might constantly throw "*** has stopped" dialogs that effectively prevents you from using the phone.


Also, some newer ROMs have alerts in their updater-script that detect different /data formats and, if detected, throw error about it, and will not proceed unless you perform a wipe.