I guess you're talking about swank-js and swank-clojure? I think nrepl-mode does nothing with SLIME. I installed both via MELPA and everything just work.
swank-js and swank-clojure are two SLIME backends that implement the SLIME's protocol. swank-clojure doesn't support newer version of SLIME, and recently became deprecated. nrepl on the other hand has it's own protocol.