I tend to cleave pretty close to the prototype, with a few exceptions. Of course, selective compression is necessary to turn real-world track alignments into model layouts. I have an Athearn SW7 painted in my roadname, even though the railroad I model never operated an SW7 and it carries a number assigned to an S1. I also have a boxcar painted for a nonexistent railroad (another model railroader who made a freelanced line based on the real-world line I model.) And I managed to score on eBay a Walthers S1 painted in my roadname, but painted in a scheme my prototype did not use until the 1970s (I model the 1950s-60s.) Eventually I will probably have some "railfan special" passenger cars that are models of SN electrics that were either burned for scrap or still exist but aren't yet restored. I may also run some streetcars even though those went away before the period I model.
So, generally, I tend to play fast and loose with the motive power timeline, but other than that I stick pretty close to the history.