Regarding the previous comment, String is a Java class so it isn't a clojure.lang.Counted. Is the type hint necessary? Maybe it should be on the call rather than the defn.
Ignoring the type hinting, I'll suggest a slightly simpler way to implement the rindex logic:
(defn rindex [coll i]
(if (neg? i) (+ (count coll) i) i))
In any case, I'm not sure rindex should be public even if you want the subs and subvec enhancements. Someone needs to make the case for adding a new function to core.
The Pythonic negative index is a debatable feature since it's pretty easy to implement for yourself if you want it.