My personal opinion is that yordles are furry little creatures. Teemo, Rumble, Kennen, and Ziggs.
Then there are the non-furry blueish skinned little creatures. Lulu, Poppy, and Tristana; they arent yordles, but they are related. I call em meglings, since that used to be a thing.
there are small humans that i wouldnt say are related to yordles at all. These would be Amumu, Corki, Nunu and Annie.
Veigar and Heimer are tough to place.
Presumably, Veigar is not a yordle or a megling, since he has a much smaller build than them. so that would leave him as a little human, or something entirely different. hard to really say with his hidden, everything.
Heimer also has a much smaller build so i wouldnt place him as a yordle or megling either. He does seem somewhat humanish to me even with that ... hair? exposed brain? spaghetti?
oh, and fizz is something related to a fish