Re: Deleting households per save game
If you are on pc then i can help a bit,and i don't no how adept you are with your computer, so forgive me if i type too much.
Your save game neighborhoods are saved in documents/electronic arts/the sims 4/saves folder,the file is saved as Slot_00000001.save, everytime you create a new game and you don't delete the otd 1, the new hood would be Slot_00000002.save, i am up to slot 9,also note if you have to move your folders due to a game breaking glitch and do a clean install of the folder, it will go back to slot 1 new file again,so when you put back your old hood make sure its not the same filename, you can rename the new hood to the next number not used.
Only your created sims,houses you build and save to your library and items you download from the gallery are saved in the sims 4 'tray' folder,to use in all hoods, just in case you didn't know that.
As far as deleting all premades household sims and unplayed townies,note that the game requires special sims pending on the packs you have, like sages,so you wouldn't want to delete them,unless you are familiar how to create new ones,or else it will just spawn another sage that might look worse than the originals,(unsure about deleting premade celebrities,mermaids,if they are required). For vendors it picks any sim,when i start a new hood,since it doesn't have enough townies spawned, i see bella,eva,even teen premades as the vendor.
So if you are going to delete them all, i suggest you make just enough to fill at least vendor slots,so it won't spawn too many unwanted townies.
Good luck and enjoy