@KatyFernlily
I mean, not every commercial lots should be mix up together. I wrote example with gym/swimming pool, because I didn't know swimming pool has any buff/moodlets. I know only about gym, library, and art gallery moodlets/buffs. Consignment store and elixir shop only require cashbox, and don't have buff/moodlets (at least visible) connected with lot type, so they can be "parasitic" to almost all types of lots.
I think more important is quantity of stuff per lot and density of lots, than is't size. Game doesn't like cluttered lots close to each other. If lots are close to each other, it's better to keep them modest.
Sometimes it's better to connect certain types of lots, than creating 60 commercial lots, just to have buff/moodlets. People from tumblr create small island-worlds with 20 lots, so it's not bad, that they are making 3in1 commercial lots. At least there is everything on these worlds.