I couldn't quite work Pepe out, although I'd assumed that he was human.
Regarding Jewels "passing for a tall dwarf", Carrot is a 6ft 6inch dwarf and he is apparently accepted as such, at least by the majority ( although as per Thud!, not all )
how tall IS a dwarf, anyway? In the absence of any real information, since characters like Cheery and the other Watch dwarfs apparently use human furniture and buildings, and the dwarf tunnels, cells etc in Fifth Elephant and Thud! seem to have headroom for humans, and the Dwarf King and Troll King play at a common gaming table, I'd pictured them as being between 4ft 6 inches and 5 feet tall.
On that basis, Jewels could be say 5ft 2inches or so and be on the rights scale; or it may just be a throw-away joke that shouldn't be looked at too seriously
There's a fair amount of inconsistency, often for comic effect, but I'd always pictured Detritus as being between 7 feet and 8 feet tall ( he appears to be able to use the human-sized Pseudopolis Yard Watch House, for one thing ) although there's considerable variation among trolls, per Asphalt in Soul Music and Bluejohn the one-troll riot shield. Then again, we are told that trolls keep growing as long as they live, so there's no reason a sufficiently old troll couldn't be any size at all, although probably substantially reverted to immobility by then.
Brick the street troll appears to be able to walk in dwarf tunnels which have headroom, and given his addled state he is presumably walking more-or-less upright, which infers he is the same sort of height as Carrot