Although corys will clean/scavenge off teh bottom of the tank, you still need to keep your gravel/sand substrate very clean yourself as corys can only clean like the top 1cm of substrate and can get infections if the substrate is not clean enough. They will also need their diet substituted with catfish pelets and/or algae wafers as well, they are some of the most peaceful fish you can get and need to be kept in groups but your tank is already getting quite crowded anyways.