Bam, welcome! 66F really isn't that cold. Many tropicals like neon tetras and most danios would thrive at that temperature. In regards to a bottom feeder, emerald corydoras do well at cooler temps. Is your intent to add more fish for activity at the substrate level? There really is no such thing as a "clean up crew". Corys need to be fed sinking pellet food, as do most "bottom feeders". Adding more fish always adds more waste to your tank.