Frimlin

With Residential, they can't run the retail store, but can tend the bar, use gym gear, make drinks at the café...etc. Not perfect but okay.

As Residential, the "commercial" part ends up being a "free for all guests" kind of place?

Correct. Lot guests still need to be invited in. But once in, they can go anywhere & do anything, as long as doors aren't locked (useful!)

And you can have separate flats with separate NPC families on the same lot while still playing a family there like in default buildings?

Not really. They'd need to be in the same household & would thus be marked as Played. You can ignore them & let them live on their own!

Weird stuff. I really kinda miss some of the Sims3 building options. Could make a whole highrise with separate flats there.

Agreed. They decided to focus on making a vibrant city expansion, rather than make customisable apartments & stuff like that. Maybe future!

With spiral stairs and terrain elevation tool!! ;p