The designs look unique enough to me, but to answer your question, I think they're mostly based off Sonic and have similiar builds because many of them were designed as being speedy characters as well (Shadow, Espio, Blaze, Jet, etc.) And then so many of the characters are rivals and probably have similar builds to Sonic to highlight that sense of the rivals being "equals."

As for the so many characters having the clean Sonic muzzle, I'd say they probably use it a lot because it looks nice. They probably considered giving characters like Rouge and Charmy different muzzles, but does anyone really want to see Charmy with a fuzzy face? The Sonic muzzle just works.

This is just me coming up with stuff. They probably do it because these types of designs are easily recognizable.