Although not everything labeled "HO scale" or "1:87 scale" is truly to scale, as a rule, those items will "fit in."
Matchbox cars were originally scaled to fit the box, hence the name Matchbox cars. Hot wheels and other brands of similar toys were designed the same way.
Years ago, verious German manufacturers such as Roco and Wiking were the only large suppliers of a wide array of HO scale vehicles. They were fine for most American modelers, if you could live with having such a large proportion of European vehicles on your layout.
Today, it's not really much of a problem to populate your layout with American vehicles, especially if you model the steam to diesel transition era. There are a number of manufacturers supplying HO scale vehicle models covering everything from the turn of the century to the current model year. Granted, the selection can be a little thin for some eras, especially the 70s and 80s, but enough models are available to have a good variety.
In the past two years or so, Wal-Mart, and to a limited extent some other retailers like Hobby Lobby and Kay Bee Toys, has begun carrying a line of HO scale vehicles under the brand names Malibu and Fresh Cherries. Athearn has a line of very nice tractor trailer models as well as Coca-Cola and John Deere branded vehicles, Norscot offers a selection of Caterpillar construction equipment and Walthers and Atlas have offered various vehicle models over the years.
I am a vehicle modeler as much as a railroad modeler, having chosen HO scale as the common denominator between the two interests, and I feel confident in saying that now is the best time to pursue this hobby. The selection is vast, so if you want to have models that fit in much better than Matchbox and Hot Wheels collectibles, pay a visit to your local hobby shop or these websites:
http://www.truckstopmodels.com/
http://www.walthers.com/exec/search...=&keywords=&instock=Q&split=300&Submit=Search
http://www.athearn.com/Search/Defau...rch=Type+Search+Terms+Here&x=0&y=0&txtSearch=