I am not sure I am hot on the startup vs small company distinction, because there exist so many examples of cross-overs. First, there are a lot of tech "startups" (meeting pg's definition) that have preferred to stay small, becoming "lifestyle businesses" as the pejorative term has it (no, it is not my preferred term!). I bet I can think of many that started out as startups, but would not meet pg's definition of startup anymore; there may even be a few YC-backed companies in that list. The opposite also occurs and has occurred, a non-scalable-model based small business morphing into a scalable company.

At some level, all taxonomies break down, but startup vs small business taxonomy isn't very strong to begin with.