So, as the lowest pinned comment says, the mod is customizable.

To get the population both realistic AND not have it be hard to get those later milestones (if you're playing with milestones) I've figured out a good system (at least with vanilla buildings).

1: don't change almost any 2x2 buildings from single home, especially if they're 1 story. This also applies to 1x1 and 1x2.

2: if it's a 2x3 or 3x3 2-story, if it looks like a town-house (2 story with slopped roof, not that wide) set it to duplex, if it looks more like an apartment (shorter roof, wide, lots of small windows) set it to either vanilla or European apartments depending on whether or not it looks like it should have >5 families.

3: if it's a 2x3 or 3x3 3-story, set it to either vanilla or European apartments (modern) depending on whether or not it looks like it should have >5 families.

3-a: if it's a 2x3 or 3x3 3-story with large windows in a more modern style, set it to either duplex or vanilla (richer people live there so they get extra room, European apartment settings have too many families)

4: if it's a lvl 3+ 3x3, 3x4 or 4x4 1 story that fills up most of the space (square) set it to duplex (again richer people live there so they get a lot of room)

4-a: if 3x3, 3x4, 4x4 1 story that fills up most of the space that also has a patio in the center, and is even more square, maybe set it to vanilla.

5: if it's a 3x4 or 4x4 2 or 3-story that fills up part of the space (L shaped, Z shaped) and looks like a multi-family mansion, set it to either duplex or vanilla (again, European apartment settings have too many families)

5-a: Finally, if it's a 3x4 or 4x4 2 or 3-story that fills up all the space (Square or mostly square) especially if it looks like an apartment, just set it to European apartments (modern).

The whole process on vanilla might take roughly 8+ minutes with a few 2nd checks if you find any houses that don't look right in terms of families living there.

There is a search bar in the mod options (alt-E) in the top right corner, search for 3x, 4x, and then 2x.