The size of passengers depends on what passenger cars you are using. The K-Line passengers are made to fit K-Line scale cars, which have large seats. Since most of my passenger fleet is K-Line, they work fine, as do the RMT remakes. I use silicone adhesive. The seats in MTH cars are smaller and so are their figures. I have a few sets of MTH cars and I bought a bag of RK passengers to fit them, but I haven't gotten around to painting them yet.
I sometimes touch up the paint on the K-Line figures to keep them from looking so much the same. Changing a blouse from green to red or a suit jacket from blue to tan is easy and creates a bit more variety. It really doesn't make that much difference, though - you don't see all that much through the windows. The Chinese purple pants are ghastly but you can't usually see them when they are in their seats.
I also sometimes play with the hair and skin tones on Standard Gauge passengers, which are big enough that you can really see the difference. Some of my Standard Gauge passenger cars have hinged roofs to show off the interiors.
