I believe this is because the return value of
'xorg-configuration-file' is a monadic value, and thus it needs to
"unwrapped" in order to pass the configuration file to
'xorg-start-command'.

Try this:

    (mlet %store-monad ((config
                         (xorg-configuration-file #:drivers '("radeon" "vesa")
                                                  #:resolutions '((1366 768)
                                                                  (1024 768)))))
      (slim-service #:startx (xorg-start-command #:configuration-file config)))