How about shrimps?
I don't know about the cory, but I'm thinking ottos would use up less space/bio load, assuming both are in schools. However, ottos, from what I've heard/ read, are known to be sensitive to water conditions and sudden deaths. I know someone whose ottos all died. But with mine, I didn't really give them any special care, and had even put them in a semicrowded tank for a while, and all are still alive (few months now). Just made sure they had algae, wafers and cucumbers to eat.