Getting more will certainly promote more activity, but you are right that most Corys like water in the low to high 70s. There are some warmer water species out there, but the question is if they are available. Planetcatfish.com is the best source of catfish information, and they have some good search functions, such as being able to search for fish by size, temperature, etc. you're going to want to look for Corys that come from Venezuela, Colombia, and other norther countries in South America.