I have quite a few Arttista figures but a couple of others with some interesting figures are Metal Figures (.net) and Woodland Scenics. Metal Figures sells individual figures and I like the half-bodied drivers they make to put in vehicles and the kids flying kites. I also bought a great funeral set from them of pallbearers carrying a casket. Woodland Scenics have sets of plastic people that have nice detail and typically come 5 to a set at a decent price. All sorts of other people out there including cheap sets of plastic people on auction site that are great if you want lots of people for little money. Have used those to create a crowd in front of a movie theatre, and butchered some of them (cut off feet, legs, et al) to use them inside of passenger trains or vehicles.
While everything I mentioned above is labeled O scale, they aren't all the same size. Such is humanity, so I wouldn't sweat it. I even have some Homies for certain scenes and those clearly aren't O scale (bigger) but fit in if grouped together.
I agree that the 1:43 model cars can be too big depending on the model. Likewise, some 1:50 model cars just look too small if your trains are scale. The display of cars you see at the end of the aisle in a drugstore seem like they'd be a good fit when you're in the store, but some of those are even bigger than 1:43. I bought a few thinking they would look good but wound up giving those cars to my nephew's boys.
