This may sound dumb, but I feel that using ponies is probably less efficient than using a full sized horse.
https://web.archive.org/web/20120127105354/http://www.postalmuseum.si.edu/resources/6a2b_ponyexpress.html
No, but just barely. The average Pony Express horse was 14 1/2 hands high, and 14.2 is generally regarded as the cutoff between the two.