Because i have some cores that dont work well i have to clear my data and start over again 
But what happens if i backup my core specific config files, clear my data an place my config back? Will they work ? will they screw everything up?
They should work fine. You shouldnât need to clear out the cores, though, unless you just donât like looking at them in the list. You can go to settings > playlists and change the core associated with a given playlist.
Except that doesnât work if youâve already tried using a different core. Once youâve played a game with one core, it writes that into the playlist which then overrides whatever you have it set to in the playlist settings. The only way Iâve found to be able to test a game with different cores without constantly editing the playlist is to set the core entries back to âDETECTâ and then set the whole playlist to read-only to prevent RA from writing to it.
When selecting a game in the menu, there really should be a âchoose default coreâ option that will allow you to change whatever core has been written into the playlist file without having to edit it manually.