Plus, since the show's creators don't specifically plan the population size for the town, and just want the town to look not empty (or empty when the episode's scenario demands that), then using different estimation methods WILL almost always give VERY different results.
They would be only similiar if the creators wanted to be realistic, or if we were analyzing an actual town.
Some people simply counted the number of houses visible in the theme song (plus estimated the number of houses off-screen), multiplied the number by 4 or a bit less/more (ie. assumed number of ponies living in each house), and ended up with various values below 1000.