Corydoras will eat algae, but not much. Mine love algae wafers but they don't munch on any of the algae elsewhere in the tank. It's not good for their diet though, they need some meat. Mine are on a diet of brine shrimp and shrimp pellets. They eat what my bettas eat.
Otos eat algae, but they are very sensitive. I hear storys of them often not surviving long. They are caught wild.
For my tanks I opted for shrimp to eat the algae build up. Some shrimp are better than others. I have ghost shrimp who will go after fish food over algae. Amano shrimp might be good. Cherry shrimp are really good, but a betta might go after a cherry (depends on the betta). Adult shrimp usually do better with bettas, as the bettas would prefer to eat anything small enough. I use nerites to keep the glass clean, they do an excellent job.