you should NOT load any core before choosing your game (ie : if you launch scummVM interface from Retroarch, you won't be able to launch any game, retroarch's build of scummvm can't work properly).
you're supposed to choose a game, then retroarch will ask you which core you want to use to launch it. And it will work perfectly fine this way.
(and for scummVM you should make .scummvm file for each game...or use retroarch scan to make playlists)