SAE's are good and fine but of course they eat algae, bio-film and stuff of that nature. Cory's don't eat much algae, they'll nibble on some but they aren't algae eaters. They are scavenger's much like Ghost Shrimp and for a happy shoal of cory's it depends on which ones. I know for my Dainty cory's (Corydoras Habrosus and for Corydoras Hastasus) you should have about 9-10 of them. For the bigger ones, Julii's will like 6+ and most other cory's will be the same. You can keep a trio, but more is definitely welcome in their group!
RJ (H. Lifalili), Acara (EB)
Ghost, Tuxedo, Trip (Angels)
Leopards: 1:3 Hawkeye, Luna, Astrael, Dorian