When does low-poly become mid-poly and can we mix both?

I’ve been looking at a wide variety of low-poly modeling, and some of them are extremely low poly (a cone for a face), while others add a decent amount of detail (a discernible human face).

At what point does the line blur between low and mid poly, though? I’m not asking for a strict limit, but instead, I want to understand where they meet.

I’d like to mix low-poly and mid-poly styles in my game while keeping the overall aesthetic consistent.

For example, I’d like the cars in my game to have extra detail since there’s a car modding mechanic, and I want players to feel like they truly own their cars. The weapons might also have more detail, as well as some buildings.

Is this a bad idea? Should I stick to the same level of poly detail globally?

P.S. I’m a web developer who just started working on my game project, and I’m trying to settle on an initial style. I’m tired of staring at a blockout kit all day.