payn
Glory to Marik
This is a good framing. Some folks might be thinking SW would be a weird story if it was Luke Skywalker and 4 wookies. If you are looking for that wookie or Groot kind of rarity of story, then I can see limiting how many exotic PCs there are.There is a difference between rare in the world and rare in the story. Jedi are rare in the SW galaxy but ever present in the stories we see.
If you want gnomes rare in your world you can still have a party full of them in your story (see hobbits at the beginning of LotR). You can also have things not rare in the world but rare in the story (see humans in Gravity).
So the answer to the question about gnome rarity is to ask if you are talking world or story being told?
As said previously, have the gnomes in the entire history of the world never teamed up with each other on adventures? That's just odd for a normally procreated social being.