Well my Cory Catfish lived with a Goldfish and a Pleco for over a year now but not she's with 3 of her kind as well as my female betta Chie, so its not like she'll die or anything but it is much better for her health and well being, its just not good to keep them alone.
Those darn Pet Store Workers, that's how they got me to take home a Pleco and a Cory Catfish!