It seems like you're already at capacity for stocking with that number of fish in a 20 gallon. Do you have another tank that you can use to switch out some of these fish? Maybe you could get more of the fish that should be in groups and split them between two tanks?
Emperor tetras should be in groups of 5 or more
CPDs should be in a gorup of 5 or more
Corys should be in a group of 4-5 or more
Animal testing is a terrible idea; they get all nervous and give the wrong answers.