"Scale doesn't really apply to a lot of these passenger cars. It depends on what fits, and what looks best. Most of the time, the legs or at least feet, get chopped off. I keep a variety of figures on hand from different manufacturers and different scales".
"a 1/48 figure does not always fit or look right in q 1/48 car. I have used 1/50, 1/64, and even 1/75 figures for domes. Their heads should not be jammed into the glass, nor should their shoulders tower above the head rests "
This is very good advice, nicely stated. I don't use "soft" adhesives i.e. silicone or hot glue. Consider storage conditions that may be hot and dry. Stuff dries out and the people are in the aisle ways. I don't like opening up passenger cars so I went for a more permanent solution like CA gel. Also scraping the paint and chopping the feet is inevitable. Disregard this if you think you are going to want to replace or remove people, although a short, sharp, shock will even dislodge CA bonding (i.e. engineers in dummy units).
Just my 2 cents!
Rich